Method of optimizing a metrology process

ABSTRACT

Methods of optimizing a metrology process are disclosed. In one arrangement, measurement data from a plurality of applications of the metrology process to a first target on a substrate are obtained. Each application of the metrology process includes illuminating the first target with a radiation spot and detecting radiation redirected by the first target. The applications of the metrology process include applications at a) plural positions of the radiation spot relative to the first target, and/or b) plural focus heights of the radiation spot. The measurement data includes, for each application of the metrology process, a detected pupil representation of an optical characteristic of the redirected radiation in a pupil plane. The method includes determining an optimal alignment and/or an optimal focus height based on comparisons between the detected pupil representations in the measurement data and a reference pupil representation.

This application claims the benefit of priority of European patentapplication no. EP18154885, filed Feb. 2, 2018, which is incorporatedherein in its entirety by reference.

FIELD

The present disclosure relates to optimizing a metrology process,particularly in relation to alignment and/or focus of a radiation spotused by the metrology process.

BACKGROUND

A lithographic apparatus is a machine that applies a desired patternonto a substrate, usually onto a target portion of the substrate. Alithographic apparatus can be used, for example, in the manufacture ofintegrated circuits (ICs). In that instance, a patterning device, whichis alternatively referred to as a mask or a reticle, may be used togenerate a circuit pattern to be formed on an individual layer of theIC. This pattern can be transferred onto a target portion (e.g.,including part of, one, or several dies) on a substrate (e.g., a siliconwafer). Transfer of the pattern is typically via imaging onto a layer ofradiation-sensitive material (resist) provided on the substrate. Ingeneral, a single substrate will contain a network of adjacent targetportions that are successively patterned.

Manufacturing devices, such as semiconductor devices, typically involvesprocessing a substrate (e.g., a semiconductor wafer) using a number offabrication processes to form various features and often multiple layersof the devices. Such layers and/or features are typically manufacturedand processed using, e.g., deposition, lithography, etch,chemical-mechanical polishing, and ion implantation. Multiple devicesmay be fabricated on a plurality of dies on a substrate and thenseparated into individual devices. This device manufacturing process maybe considered a patterning process. A patterning process involves apattern transfer step, such as optical and/or nanoimprint lithographyusing a lithographic apparatus, to provide a pattern on a substrate andtypically, but optionally, involves one or more related patternprocessing steps, such as resist development by a development apparatus,baking of the substrate using a bake tool, etching the pattern by anetch apparatus, etc. Further, one or more metrology processes areinvolved in the patterning process.

Metrology processes are used at various steps during a patterningprocess to monitor and/or control the process. For example, metrologyprocesses are used to measure one or more characteristics of asubstrate, such as a relative location (e.g., registration, overlay,alignment, etc.) or dimension (e.g., line width, critical dimension(CD), thickness, etc.) of features formed on the substrate during thepatterning process, such that, for example, the performance of thepatterning process can be determined from the one or morecharacteristics. If the one or more characteristics are unacceptable(e.g., out of a predetermined range for the characteristic(s)), one ormore variables of the patterning process may be designed or altered,e.g., based on the measurements of the one or more characteristics, suchthat substrates manufactured by the patterning process have anacceptable characteristic(s).

With the advancement of lithography and other patterning processtechnologies, the dimensions of functional elements have continuallybeen reduced while the amount of the functional elements, such astransistors, per device has been steadily increased over decades. In themeanwhile, the requirement of accuracy in terms of overlay, criticaldimension (CD), etc. has become more and more stringent. Error, such aserror in overlay, error in CD, etc., will inevitably be produced in thepatterning process. For example, imaging error may be produced fromoptical aberration, patterning device heating, patterning device error,and/or substrate heating and can be characterized in terms of, e.g.,overlay, CD, etc. Additionally or alternatively, error may be introducedin other parts of the patterning process, such as in etch, development,bake, etc. and similarly can be characterized in terms of, e.g.,overlay, CD, etc. The error may cause a problem in terms of thefunctioning of the device, including failure of the device to functionor one or more electrical problems of the functioning device.Accordingly, it is desirable to be able to characterize one or more ofthese errors and take steps to design, modify, control, etc. apatterning process to reduce or minimize one or more of these errors

Various tools are available for performing metrology processes,including various forms of scatterometer. These devices direct a beam ofradiation onto a metrology target and measure one or more properties ofthe scattered radiation—e.g., intensity at a single angle of reflection,or over a range of angles of reflection, as a function of wavelength;intensity at one or more wavelengths as a function of reflected angle;or polarization as a function of reflected angle—to obtain a “spectrum”from which a property of interest of the target can be determined.Determination of the property of interest may be performed by varioustechniques: e.g., reconstruction of the metrology target by iterativeapproaches implemented using rigorous coupled wave analysis or finiteelement methods; library searches; and principal component analysis.

SUMMARY

It may be desirable for metrology targets to be positioned in locationswhere there is little space available, for example in product areascontaining structures of a product being manufactured. Metrology targetspositioned in such areas need to be small. It is challenging to align aradiation spot with such metrology targets with sufficient accuracy. Ifalignment is not perfect, the radiation spot may sample regions outsideof the metrology target, thereby reducing an accuracy of the metrologyprocess.

It is desirable to improve existing methods for measuring targets.

According to an aspect of the invention, there is provided a method ofoptimizing a metrology process, the method comprising:

obtaining measurement data from a plurality of applications of themetrology process to a first target on a substrate, wherein: eachapplication of the metrology process comprises illuminating the firsttarget with a radiation spot and detecting radiation redirected by thefirst target; the applications of the metrology process includeapplications at either or both of a) plural positions of the radiationspot relative to the first target, and/or b) plural focus heights of theradiation spot; and the measurement data comprises, for each applicationof the metrology process, a detected pupil representation of an opticalcharacteristic of the redirected radiation in a pupil plane; and

determining either or both of an optimal alignment and/or an optimalfocus height based on comparisons between the detected pupilrepresentations in the measurement data and a reference pupilrepresentation.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described, by way of exampleonly, with reference to the accompanying schematic drawings in whichcorresponding reference symbols indicate corresponding parts, and inwhich:

FIG. 1 schematically depicts an embodiment of a lithographic apparatus;

FIG. 2 schematically depicts an embodiment of a lithographic cell orcluster;

FIG. 3A is a schematic diagram of a measurement apparatus for use inmeasuring targets according to an embodiment using a first pair ofillumination apertures providing certain illumination modes;

FIG. 3B is a schematic detail of a diffraction spectrum of a target fora given direction of illumination;

FIG. 3C is a schematic illustration of a second pair of illuminationapertures providing further illumination modes in using a measurementapparatus for diffraction based overlay measurements;

FIG. 3D is a schematic illustration of a third pair of illuminationapertures combining the first and second pairs of apertures providingfurther illumination modes in using a measurement apparatus fordiffraction based overlay measurements;

FIG. 4 schematically depicts a form of multiple periodic structure(e.g., multiple grating) target and an outline of a measurement spot ona substrate;

FIG. 5 schematically depicts an image of the target of FIG. 4 obtainedin the apparatus of FIG. 3;

FIG. 6 schematically depicts an example of a metrology apparatus andmetrology technique;

FIG. 7 schematically depicts an example of a metrology apparatus;

FIG. 8 illustrates the relationship between an illumination spot of ametrology apparatus and a metrology target;

FIG. 9 schematically depicts a process of deriving one or more variablesof interest based on measurement data;

FIG. 10A schematically depicts an example unit cell, an associated pupilrepresentation, and an associated derived pupil representation;

FIG. 10B schematically depicts an example unit cell, an associated pupilrepresentation, and an associated derived pupil representation;

FIG. 10C schematically depicts an example target comprising one or morephysical instances of a unit cell;

FIG. 11 schematically depicts device regions and scribe lanes on asubstrate;

FIG. 12 is a flow chart depicting a method of generating measurementdata;

FIG. 13 is a flow chart depicting a method of optimizing a metrologyprocess according to an embodiment;

FIG. 14 depicts example measurement data;

FIG. 15 is a graph showing variation of a correlation between a detectedpupil representation and a reference pupil representation as a functionof alignment; and

FIG. 16 depicts a computer system which may implement embodiments of thedisclosure.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

Before describing embodiments in detail, it is instructive to present anexample environment in which embodiments may be implemented.

FIG. 1 schematically depicts a lithographic apparatus LA. The apparatuscomprises:

-   -   an illumination system (illuminator) IL configured to condition        a radiation beam B (e.g. UV radiation or DUV radiation);    -   a support structure (e.g. a mask table) MT constructed to        support a patterning device (e.g. a mask) MA and connected to a        first positioner PM configured to accurately position the        patterning device in accordance with certain parameters;    -   a substrate table (e.g. a wafer table) WT constructed to hold a        substrate (e.g. a resist-coated wafer) W and connected to a        second positioner PW configured to accurately position the        substrate in accordance with certain parameters; and    -   a projection system (e.g. a refractive projection lens system)        PS configured to project a pattern imparted to the radiation        beam B by patterning device MA onto a target portion C (e.g.        comprising one or more dies) of the substrate W, the projection        system supported on a reference frame (RF).

The illumination system may include various types of optical components,such as refractive, reflective, magnetic, electromagnetic, electrostaticor other types of optical components, or any combination thereof, fordirecting, shaping, or controlling radiation.

The support structure supports the patterning device in a manner thatdepends on the orientation of the patterning device, the design of thelithographic apparatus, and other conditions, such as for examplewhether or not the patterning device is held in a vacuum environment.The support structure can use mechanical, vacuum, electrostatic or otherclamping techniques to hold the patterning device. The support structuremay be a frame or a table, for example, which may be fixed or movable asrequired. The support structure may ensure that the patterning device isat a desired position, for example with respect to the projectionsystem. Any use of the terms “reticle” or “mask” herein may beconsidered synonymous with the more general term “patterning device.”

The term “patterning device” used herein should be broadly interpretedas referring to any device that can be used to impart a pattern in atarget portion of the substrate. In an embodiment, a patterning deviceis any device that can be used to impart a radiation beam with a patternin its cross-section so as to create a pattern in a target portion ofthe substrate. It should be noted that the pattern imparted to theradiation beam may not exactly correspond to the desired pattern in thetarget portion of the substrate, for example if the pattern includesphase-shifting features or so called assist features. Generally, thepattern imparted to the radiation beam will correspond to a particularfunctional layer in a device being created in the target portion, suchas an integrated circuit.

The patterning device may be transmissive or reflective. Examples ofpatterning devices include masks, programmable mirror arrays, andprogrammable Liquid Crystal Display (LCD) panels. Masks are well knownin lithography, and include mask types such as binary, alternatingphase-shift, and attenuated phase-shift, as well as various hybrid masktypes. An example of a programmable mirror array employs a matrixarrangement of small mirrors, each of which can be individually tiltedso as to reflect an incoming radiation beam in different directions. Thetilted mirrors impart a pattern in a radiation beam, which is reflectedby the mirror matrix.

The term “projection system” used herein should be broadly interpretedas encompassing any type of projection system, including refractive,reflective, catadioptric, magnetic, electromagnetic and electrostaticoptical systems, or any combination thereof, as appropriate for theexposure radiation being used, or for other factors such as the use ofan immersion liquid or the use of a vacuum. Any use of the term“projection lens” herein may be considered as synonymous with the moregeneral term “projection system”.

The projection system PS has an optical transfer function which may benon-uniform, which can affect the pattern imaged on the substrate W. Forunpolarized radiation such effects can be fairly well described by twoscalar maps, which describe the transmission (apodization) and relativephase (aberration) of radiation exiting the projection system PS as afunction of position in a pupil plane thereof. These scalar maps, whichmay be referred to as the transmission map and the relative phase map,may be expressed as a linear combination of a complete set of basisfunctions. A particularly convenient set is the Zernike polynomials,which form a set of orthogonal polynomials defined on a unit circle. Adetermination of each scalar map may involve determining thecoefficients in such an expansion. Since the Zernike polynomials areorthogonal on the unit circle, the Zernike coefficients may bedetermined by calculating the inner product of a measured scalar mapwith each Zernike polynomial in turn and dividing this by the square ofthe norm of that Zernike polynomial.

The transmission map and the relative phase map are field and systemdependent. That is, in general, each projection system PS will have adifferent Zernike expansion for each field point (i.e. for each spatiallocation in its image plane). The relative phase of the projectionsystem PS in its pupil plane may be determined by projecting radiation,for example from a point-like source in an object plane of theprojection system PS (i.e. the plane of the patterning device MA),through the projection system PS and using a shearing interferometer tomeasure a wavefront (i.e. a locus of points with the same phase). Ashearing interferometer is a common path interferometer and therefore,advantageously, no secondary reference beam is required to measure thewavefront. The shearing interferometer may comprise a diffractiongrating, for example a two dimensional grid, in an image plane of theprojection system (i.e. the substrate table WT) and a detector arrangedto detect an interference pattern in a plane that is conjugate to apupil plane of the projection system PS. The interference pattern isrelated to the derivative of the phase of the radiation with respect toa coordinate in the pupil plane in the shearing direction. The detectormay comprise an array of sensing elements such as, for example, chargecoupled devices (CCDs).

The projection system PS of a lithography apparatus may not producevisible fringes and therefore the accuracy of the determination of thewavefront can be enhanced using phase stepping techniques such as, forexample, moving the diffraction grating. Stepping may be performed inthe plane of the diffraction grating and in a direction perpendicular tothe scanning direction of the measurement. The stepping range may be onegrating period, and at least three (uniformly distributed) phase stepsmay be used. Thus, for example, three scanning measurements may beperformed in the y-direction, each scanning measurement being performedfor a different position in the x-direction. This stepping of thediffraction grating effectively transforms phase variations intointensity variations, allowing phase information to be determined. Thegrating may be stepped in a direction perpendicular to the diffractiongrating (z direction) to calibrate the detector.

The transmission (apodization) of the projection system PS in its pupilplane may be determined by projecting radiation, for example from apoint-like source in an object plane of the projection system PS (i.e.the plane of the patterning device MA), through the projection system PSand measuring the intensity of radiation in a plane that is conjugate toa pupil plane of the projection system PS, using a detector. The samedetector as is used to measure the wavefront to determine aberrationsmay be used.

The projection system PS may comprise a plurality of optical (e.g.,lens) elements and may further comprise an adjustment mechanism AMconfigured to adjust one or more of the optical elements so as tocorrect for aberrations (phase variations across the pupil planethroughout the field). To achieve this, the adjustment mechanism may beoperable to manipulate one or more optical (e.g., lens) elements withinthe projection system PS in one or more different ways. The projectionsystem may have a coordinate system wherein its optical axis extends inthe z direction. The adjustment mechanism may be operable to do anycombination of the following: displace one or more optical elements;tilt one or more optical elements; and/or deform one or more opticalelements. Displacement of an optical element may be in any direction (x,y, z or a combination thereof). Tilting of an optical element istypically out of a plane perpendicular to the optical axis, by rotatingabout an axis in the x and/or y directions although a rotation about thez axis may be used for a non-rotationally symmetric aspherical opticalelement. Deformation of an optical element may include a low frequencyshape (e.g. astigmatic) and/or a high frequency shape (e.g. free formaspheres). Deformation of an optical element may be performed forexample by using one or more actuators to exert force on one or moresides of the optical element and/or by using one or more heatingelements to heat one or more selected regions of the optical element. Ingeneral, it may not be possible to adjust the projection system PS tocorrect for apodization (transmission variation across the pupil plane).The transmission map of a projection system PS may be used whendesigning a patterning device (e.g., mask) MA for the lithographyapparatus LA. Using a computational lithography technique, thepatterning device MA may be designed to at least partially correct forapodization.

As here depicted, the apparatus is of a transmissive type (e.g.employing a transmissive mask). Alternatively, the apparatus may be of areflective type (e.g. employing a programmable mirror array of a type asreferred to above, or employing a reflective mask).

The lithographic apparatus may be of a type having two (dual stage) ormore tables (e.g., two or more substrate tables WTa, WTb, two or morepatterning device tables, a substrate table WTa and a table WTb belowthe projection system without a substrate that is dedicated to, forexample, facilitating measurement, and/or cleaning, etc.). In such“multiple stage” machines the additional tables may be used in parallel,or preparatory steps may be carried out on one or more tables while oneor more other tables are being used for exposure. For example, alignmentmeasurements using an alignment sensor AS and/or level (height, tilt,etc.) measurements using a level sensor LS may be made.

The lithographic apparatus may also be of a type wherein at least aportion of the substrate may be covered by a liquid having a relativelyhigh refractive index, e.g. water, so as to fill a space between theprojection system and the substrate. An immersion liquid may also beapplied to other spaces in the lithographic apparatus, for example,between the patterning device and the projection system. Immersiontechniques are well known in the art for increasing the numericalaperture of projection systems. The term “immersion” as used herein doesnot mean that a structure, such as a substrate, must be submerged inliquid, but rather only means that liquid is located between theprojection system and the substrate during exposure.

Referring to FIG. 1, the illuminator IL receives a radiation beam from aradiation source SO. The source and the lithographic apparatus may beseparate entities, for example when the source is an excimer laser. Insuch cases, the source is not considered to form part of thelithographic apparatus and the radiation beam is passed from the sourceSO to the illuminator IL with the aid of a beam delivery system BDcomprising, for example, suitable directing mirrors and/or a beamexpander. In other cases the source may be an integral part of thelithographic apparatus, for example when the source is a mercury lamp.The source SO and the illuminator IL, together with the beam deliverysystem BD if required, may be referred to as a radiation system.

The illuminator IL may comprise an adjuster AD configured to adjust theangular intensity distribution of the radiation beam. Generally, atleast the outer and/or inner radial extent (commonly referred to asσ-outer and σ-inner, respectively) of the intensity distribution in apupil plane of the illuminator can be adjusted. In addition, theilluminator IL may comprise various other components, such as anintegrator IN and a condenser CO. The illuminator may be used tocondition the radiation beam, to have a desired uniformity and intensitydistribution in its cross-section.

The radiation beam B is incident on the patterning device (e.g., mask)MA, which is held on the support structure (e.g., mask table) MT, and ispatterned by the patterning device. Having traversed the patterningdevice MA, the radiation beam B passes through the projection system PS,which focuses the beam onto a target portion C of the substrate W. Withthe aid of the second positioner PW and position sensor IF (e.g. aninterferometric device, linear encoder, 2-D encoder or capacitivesensor), the substrate table WT can be moved accurately, e.g. so as toposition different target portions C in the path of the radiation beamB. Similarly, the first positioner PM and another position sensor (whichis not explicitly depicted in FIG. 1) can be used to accurately positionthe patterning device MA with respect to the path of the radiation beamB, e.g. after mechanical retrieval from a mask library, or during ascan. In general, movement of the support structure MT may be realizedwith the aid of a long-stroke module (coarse positioning) and ashort-stroke module (fine positioning), which form part of the firstpositioner PM. Similarly, movement of the substrate table WT may berealized using a long-stroke module and a short-stroke module, whichform part of the second positioner PW. In the case of a stepper (asopposed to a scanner) the support structure MT may be connected to ashort-stroke actuator only, or may be fixed. Patterning device MA andsubstrate W may be aligned using patterning device alignment marks M1,M2 and substrate alignment marks P1, P2. Although the substratealignment marks as illustrated occupy dedicated target portions, theymay be located in spaces between target portions (these are known asscribe-lane alignment marks). Similarly, in situations in which morethan one die is provided on the patterning device MA, the patterningdevice alignment marks may be located between the dies.

The depicted apparatus could be used in at least one of the followingmodes:

-   -   1. In step mode, the support structure MT and the substrate        table WT are kept essentially stationary, while an entire        pattern imparted to the radiation beam is projected onto a        target portion C at one time (i.e. a single static exposure).        The substrate table WT is then shifted in the X and/or Y        direction so that a different target portion C can be exposed.        In step mode, the maximum size of the exposure field limits the        size of the target portion C imaged in a single static exposure.    -   2. In scan mode, the support structure MT and the substrate        table WT are scanned synchronously while a pattern imparted to        the radiation beam is projected onto a target portion C (i.e. a        single dynamic exposure). The velocity and direction of the        substrate table WT relative to the support structure MT may be        determined by the (de-)magnification and image reversal        characteristics of the projection system PS. In scan mode, the        maximum size of the exposure field limits the width (in the        non-scanning direction) of the target portion in a single        dynamic exposure, whereas the length of the scanning motion        determines the height (in the scanning direction) of the target        portion.    -   3. In another mode, the support structure MT is kept essentially        stationary holding a programmable patterning device, and the        substrate table WT is moved or scanned while a pattern imparted        to the radiation beam is projected onto a target portion C. In        this mode, generally a pulsed radiation source is employed and        the programmable patterning device is updated as required after        each movement of the substrate table WT or in between successive        radiation pulses during a scan. This mode of operation can be        readily applied to maskless lithography that utilizes        programmable patterning device, such as a programmable mirror        array of a type as referred to above.

Combinations and/or variations on the above described modes of use orentirely different modes of use may also be employed.

As shown in FIG. 2, the lithographic apparatus LA may form part of alithographic cell LC, also sometimes referred to a lithocell or cluster,which also includes apparatuses to perform pre- and post-exposureprocesses on a substrate. Conventionally these include one or more spincoaters SC to deposit one or more resist layers, one or more developersDE to develop exposed resist, one or more chill plates CH and/or one ormore bake plates BK. A substrate handler, or robot, RO picks up one ormore substrates from input/output port I/O1, I/O2, moves them betweenthe different process apparatuses and delivers them to the loading bayLB of the lithographic apparatus. These apparatuses, which are oftencollectively referred to as the track, are under the control of a trackcontrol unit TCU which is itself controlled by the supervisory controlsystem SCS, which also controls the lithographic apparatus vialithography control unit LACU. Thus, the different apparatuses can beoperated to maximize throughput and processing efficiency.

In order that a substrate that is exposed by the lithographic apparatusis exposed correctly and consistently, it is desirable to inspect anexposed substrate to measure or determine one or more properties such asoverlay (which can be, for example, between structures in overlyinglayers or between structures in a same layer that have been providedseparately to the layer by, for example, a double patterning process),line thickness, critical dimension (CD), focus offset, a materialproperty, etc. Accordingly a manufacturing facility in which lithocellLC is located also typically includes a metrology system MET whichreceives some or all of the substrates W that have been processed in thelithocell. The metrology system MET may be part of the lithocell LC, forexample it may be part of the lithographic apparatus LA.

Metrology results may be provided directly or indirectly to thesupervisory control system SCS. If an error is detected, an adjustmentmay be made to exposure of a subsequent substrate (especially if theinspection can be done soon and fast enough that one or more othersubstrates of the batch are still to be exposed) and/or to subsequentexposure of the exposed substrate. Also, an already exposed substratemay be stripped and reworked to improve yield, or discarded, therebyavoiding performing further processing on a substrate known to befaulty. In a case where only some target portions of a substrate arefaulty, further exposures may be performed only on those target portionswhich are good.

Within a metrology system MET, a metrology apparatus is used todetermine one or more properties of the substrate, and in particular,how one or more properties of different substrates vary or differentlayers of the same substrate vary from layer to layer. The metrologyapparatus may be integrated into the lithographic apparatus LA or thelithocell LC or may be a stand-alone device. To enable rapidmeasurement, it is desirable that the metrology apparatus measure one ormore properties in the exposed resist layer immediately after theexposure. However, the latent image in the resist has a lowcontrast—there is only a very small difference in refractive indexbetween the parts of the resist which have been exposed to radiation andthose which have not—and not all metrology apparatus have sufficientsensitivity to make useful measurements of the latent image. Thereforemeasurements may be taken after the post-exposure bake step (PEB) whichis customarily the first step carried out on an exposed substrate andincreases the contrast between exposed and unexposed parts of theresist. At this stage, the image in the resist may be referred to assemi-latent. It is also possible to make measurements of the developedresist image—at which point either the exposed or unexposed parts of theresist have been removed—or after a pattern transfer step such asetching. The latter possibility limits the possibilities for rework of afaulty substrate but may still provide useful information.

To enable the metrology, one or more targets can be provided on thesubstrate. In an embodiment, the target is specially designed and maycomprise a periodic structure. In an embodiment, the target is a part ofa device pattern, e.g., a periodic structure of the device pattern. Inan embodiment, the device pattern is a periodic structure of a memorydevice (e.g., a Bipolar Transistor (BPT), a Bit Line Contact (BLC), etc.structure).

In an embodiment, the target on a substrate may comprise one or more 1-Dperiodic structures (e.g., gratings), which are printed such that afterdevelopment, the periodic structural features are formed of solid resistlines. In an embodiment, the target may comprise one or more 2-Dperiodic structures (e.g., gratings), which are printed such that afterdevelopment, the one or more periodic structures are formed of solidresist pillars or vias in the resist. The bars, pillars or vias mayalternatively be etched into the substrate (e.g., into one or morelayers on the substrate).

In an embodiment, one of the parameters of interest of a patterningprocess is overlay. Overlay can be measured using dark fieldscatterometry in which the zeroth order of diffraction (corresponding toa specular reflection) is blocked, and only higher orders processed.Examples of dark field metrology can be found in PCT patent applicationpublication nos. WO 2009/078708 and WO 2009/106279, which are herebyincorporated in their entirety by reference. Further developments of thetechnique have been described in U.S. patent application publicationsUS2011-0027704, US2011-0043791 and US2012-0242970, which are herebyincorporated in their entirety by reference. Diffraction-based overlayusing dark-field detection of the diffraction orders enables overlaymeasurements on smaller targets. These targets can be smaller than theillumination spot and may be surrounded by device product structures ona substrate. In an embodiment, multiple targets can be measured in oneradiation capture.

A metrology apparatus suitable for use in embodiments to measure, e.g.,overlay is schematically shown in FIG. 3A. A target T (comprising aperiodic structure such as a grating) and diffracted rays areillustrated in more detail in FIG. 3B. The metrology apparatus may be astand-alone device or incorporated in either the lithographic apparatusLA, e.g., at the measurement station, or the lithographic cell LC. Anoptical axis, which has several branches throughout the apparatus, isrepresented by a dotted line O. In this apparatus, radiation emitted byan output 11 (e.g., a source such as a laser or a xenon lamp or anopening connected to a source) is directed onto substrate W via a prism15 by an optical system comprising lenses 12, 14 and objective lens 16.These lenses are arranged in a double sequence of a 4F arrangement. Adifferent lens arrangement can be used, provided that it still providesa substrate image onto a detector.

In an embodiment, the lens arrangement allows for access of anintermediate pupil-plane for spatial-frequency filtering. Therefore, theangular range at which the radiation is incident on the substrate can beselected by defining a spatial intensity distribution in a plane thatpresents the spatial spectrum of the substrate plane, here referred toas a (conjugate) pupil plane. In particular, this can be done, forexample, by inserting an aperture plate 13 of suitable form betweenlenses 12 and 14, in a plane which is a back-projected image of theobjective lens pupil plane. In the example illustrated, aperture plate13 has different forms, labeled 13N and 13S, allowing differentillumination modes to be selected. The illumination system in thepresent examples forms an off-axis illumination mode. In the firstillumination mode, aperture plate 13N provides off-axis illuminationfrom a direction designated, for the sake of description only, as‘north’. In a second illumination mode, aperture plate 13S is used toprovide similar illumination, but from an opposite direction, labeled‘south’. Other modes of illumination are possible by using differentapertures. The rest of the pupil plane is desirably dark as anyunnecessary radiation outside the desired illumination mode mayinterfere with the desired measurement signals.

As shown in FIG. 3B, target T is placed with substrate W substantiallynormal to the optical axis O of objective lens 16. A ray of illuminationI impinging on target T from an angle off the axis O gives rise to azeroth order ray (solid line 0) and two first order rays (dot-chainline+1 and double dot-chain line −1). With an overfilled small target T,these rays are just one of many parallel rays covering the area of thesubstrate including metrology target T and other features. Since theaperture in plate 13 has a finite width (necessary to admit a usefulquantity of radiation), the incident rays I will in fact occupy a rangeof angles, and the diffracted rays 0 and +1/−1 will be spread outsomewhat. According to the point spread function of a small target, eachorder +1 and −1 will be further spread over a range of angles, not asingle ideal ray as shown. Note that the periodic structure pitch andillumination angle can be designed or adjusted so that the first orderrays entering the objective lens are closely aligned with the centraloptical axis. The rays illustrated in FIGS. 3A and 3B are shown somewhatoff axis, purely to enable them to be more easily distinguished in thediagram. At least the 0 and +1 orders diffracted by the target onsubstrate W are collected by objective lens 16 and directed back throughprism 15.

Returning to FIG. 3A, both the first and second illumination modes areillustrated, by designating diametrically opposite apertures labeled asnorth (N) and south (S). When the incident ray I is from the north sideof the optical axis, that is when the first illumination mode is appliedusing aperture plate 13N, the +1 diffracted rays, which are labeled+1(N), enter the objective lens 16. In contrast, when the secondillumination mode is applied using aperture plate 13S the −1 diffractedrays (labeled −1(S)) are the ones which enter the lens 16. Thus, in anembodiment, measurement results are obtained by measuring the targettwice under certain conditions, e.g., after rotating the target orchanging the illumination mode or changing the imaging mode to obtainseparately the −1st and the +1st diffraction order intensities.Comparing these intensities for a given target provides a measurement ofasymmetry in the target, and asymmetry in the target can be used as anindicator of a parameter of a lithography process, e.g., overlay. In thesituation described above, the illumination mode is changed.

A beam splitter 17 divides the diffracted beams into two measurementbranches. In a first measurement branch, optical system 18 forms adiffraction spectrum (pupil plane image) of the target on first sensor19 (e.g. a CCD or CMOS sensor) using the zeroth and first orderdiffractive beams. Each diffraction order hits a different point on thesensor, so that image processing can compare and contrast orders. Thepupil plane image captured by sensor 19 can be used for focusing themetrology apparatus and/or normalizing intensity measurements. The pupilplane image can also be used for other measurement purposes such asreconstruction, as described further hereafter.

In the second measurement branch, optical system 20, 22 forms an imageof the target on the substrate W on sensor 23 (e.g. a CCD or CMOSsensor). In the second measurement branch, an aperture stop 21 isprovided in a plane that is conjugate to the pupil-plane of theobjective lens 16. Aperture stop 21 functions to block the zeroth orderdiffracted beam so that the image of the target formed on sensor 23 isformed from the −1 or +1 first order beam. Data regarding the imagesmeasured by sensors 19 and 23 are output to processor and controller PU,the function of which will depend on the particular type of measurementsbeing performed. Note that the term ‘image’ is used in a broad sense. Animage of the periodic structure features (e.g., grating lines) as suchwill not be formed, if only one of the −1 and +1 orders is present.

The particular forms of aperture plate 13 and stop 21 shown in FIG. 3are purely examples. In another embodiment, on-axis illumination of thetargets is used and an aperture stop with an off-axis aperture is usedto pass substantially only one first order of diffracted radiation tothe sensor. In yet other embodiments, 2nd, 3rd and higher order beams(not shown in FIG. 3) can be used in measurements, instead of or inaddition to the first order beams.

In order to make the illumination adaptable to these different types ofmeasurement, the aperture plate 13 may comprise a number of aperturepatterns formed around a disc, which rotates to bring a desired patterninto place. Note that aperture plate 13N or 13S are used to measure aperiodic structure of a target oriented in one direction (X or Ydepending on the set-up). For measurement of an orthogonal periodicstructure, rotation of the target through 90° and 270° might beimplemented. Different aperture plates are shown in FIGS. 3C and D. FIG.3C illustrates two further types of off-axis illumination mode. In afirst illumination mode of FIG. 3C, aperture plate 13E provides off-axisillumination from a direction designated, for the sake of descriptiononly, as ‘east’ relative to the ‘north’ previously described. In asecond illumination mode of FIG. 3C, aperture plate 13W is used toprovide similar illumination, but from an opposite direction, labeled‘west’. FIG. 3D illustrates two further types of off-axis illuminationmode. In a first illumination mode of FIG. 3D, aperture plate 13NWprovides off-axis illumination from the directions designated ‘north’and ‘west’ as previously described. In a second illumination mode,aperture plate 13SE is used to provide similar illumination, but from anopposite direction, labeled ‘south’ and ‘east’ as previously described.The use of these, and numerous other variations and applications of theapparatus are described in, for example, the prior published patentapplication publications mentioned above.

FIG. 4 depicts an example composite metrology target T formed on asubstrate. The composite target comprises four periodic structures (inthis case, gratings) 32, 33, 34, 35 positioned closely together. In anembodiment, the periodic structure layout may be made smaller than themeasurement spot (i.e., the periodic structure layout is overfilled).Thus, in an embodiment, the periodic structures are positioned closelytogether enough so that they all are within a measurement spot 31 formedby the illumination beam of the metrology apparatus. In that case, thefour periodic structures thus are all simultaneously illuminated andsimultaneously imaged on sensors 19 and 23. In an example dedicated tooverlay measurement, periodic structures 32, 33, 34, 35 are themselvescomposite periodic structures (e.g., composite gratings) formed byoverlying periodic structures, i.e., periodic structures are patternedin different layers of the device formed on substrate W and such that atleast one periodic structure in one layer overlays at least one periodicstructure in a different layer. Such a target may have outer dimensionswithin 20 μm×20 μm or within 16 μm×16 μm. Further, all the periodicstructures are used to measure overlay between a particular pair oflayers. To facilitate a target being able to measure more than a singlepair of layers, periodic structures 32, 33, 34, 35 may have differentlybiased overlay offsets in order to facilitate measurement of overlaybetween different layers in which the different parts of the compositeperiodic structures are formed. Thus, all the periodic structures forthe target on the substrate would be used to measure one pair of layersand all the periodic structures for another same target on the substratewould be used to measure another pair of layers, wherein the differentbias facilitates distinguishing between the layer pairs.

Returning to FIG. 4, periodic structures 32, 33, 34, 35 may also differin their orientation, as shown, so as to diffract incoming radiation inX and Y directions. In one example, periodic structures 32 and 34 areX-direction periodic structures with biases of +d, −d, respectively.Periodic structures 33 and 35 may be Y-direction periodic structureswith offsets +d and −d respectively. While four periodic structures areillustrated, another embodiment may include a larger matrix to obtaindesired accuracy. For example, a 3×3 array of nine composite periodicstructures may have biases −4d, −3d, −2d, −d, 0, +d, +2d, +3d, +4d.Separate images of these periodic structures can be identified in animage captured by sensor 23.

FIG. 5 shows an example of an image that may be formed on and detectedby the sensor 23, using the target of FIG. 4 in the apparatus of FIG. 3,using the aperture plates 13NW or 13SE from FIG. 3D. While the sensor 19cannot resolve the different individual periodic structures 32 to 35,the sensor 23 can do so. The dark rectangle represents the field of theimage on the sensor, within which the illuminated spot 31 on thesubstrate is imaged into a corresponding circular area 41. Within this,rectangular areas 42-45 represent the images of the periodic structures32 to 35. The target can be positioned in among device product features,rather than or in addition to in a scribe lane. If the periodicstructures are located in device product areas, device features may alsobe visible in the periphery of this image field. Processor andcontroller PU processes these images using pattern recognition toidentify the separate images 42 to 45 of periodic structures 32 to 35.In this way, the images do not have to be aligned very precisely at aspecific location within the sensor frame, which greatly improvesthroughput of the measuring apparatus as a whole.

Once the separate images of the periodic structures have beenidentified, the intensities of those individual images can be measured,e.g., by averaging or summing selected pixel intensity values within theidentified areas. Intensities and/or other properties of the images canbe compared with one another. These results can be combined to measuredifferent parameters of the lithographic process. Overlay performance isan example of such a parameter.

In an embodiment, one of the parameters of interest of a patterningprocess is feature width (e.g., CD). FIG. 6 depicts a highly schematicexample metrology apparatus (e.g., a scatterometer) that can enablefeature width determination. It comprises a broadband (white light)radiation projector 2 which projects radiation onto a substrate W. Theredirected radiation is passed to a spectrometer detector 4, whichmeasures a spectrum 10 (intensity as a function of wavelength) of thespecular reflected radiation, as shown, e.g., in the graph in the lowerleft. From this data, the structure or profile giving rise to thedetected spectrum may be reconstructed by processor PU, e.g. by RigorousCoupled Wave Analysis and non-linear regression or by comparison with alibrary of simulated spectra as shown at the bottom right of FIG. 6. Ingeneral, for the reconstruction the general form of the structure isknown and some variables are assumed from knowledge of the process bywhich the structure was made, leaving only a few variables of thestructure to be determined from the measured data. Such a metrologyapparatus may be configured as a normal-incidence metrology apparatus oran oblique-incidence metrology apparatus. Moreover, in addition tomeasurement of a parameter by reconstruction, angle resolvedscatterometry is useful in the measurement of asymmetry of features inproduct and/or resist patterns. A particular application of asymmetrymeasurement is for the measurement of overlay, where the targetcomprises one set of periodic features superimposed on another. Theconcepts of asymmetry measurement in this manner are described, forexample, in U.S. patent application publication US2006-066855, which isincorporated herein in its entirety.

FIG. 7 illustrates an example of a metrology apparatus 100 suitable foruse in embodiments of the present disclosure. The principles ofoperation of this type of metrology apparatus are explained in moredetail in the U.S. Patent Application Nos. US 2006-033921 and US2010-201963, which are incorporated herein in their entireties byreference. An optical axis, which has several branches throughout theapparatus, is represented by a dotted line O. In this apparatus,radiation emitted by source 110 (e.g., a xenon lamp) is directed ontosubstrate W via by an optical system comprising: lens system 120,aperture plate 130, lens system 140, a partially reflecting surface 150and objective lens 160. In an embodiment these lens systems 120, 140,160 are arranged in a double sequence of a 4F arrangement. In anembodiment, the radiation emitted by radiation source 110 is collimatedusing lens system 120. A different lens arrangement can be used, ifdesired. The angular range at which the radiation is incident on thesubstrate can be selected by defining a spatial intensity distributionin a plane that presents the spatial spectrum of the substrate plane. Inparticular, this can be done by inserting an aperture plate 130 ofsuitable form between lenses 120 and 140, in a plane which is aback-projected image of the objective lens pupil plane. Differentintensity distributions (e.g., annular, dipole, etc.) are possible byusing different apertures. The angular distribution of illumination inradial and peripheral directions, as well as properties such aswavelength, polarization and/or coherency of the radiation, can all beadjusted to obtain desired results. For example, one or moreinterference filters 130 (see FIG. 9) can be provided between source 110and partially reflecting surface 150 to select a wavelength of interestin the range of, say, 400-900 nm or even lower, such as 200-300 nm. Theinterference filter may be tunable rather than comprising a set ofdifferent filters. A grating could be used instead of an interferencefilter. In an embodiment, one or more polarizers 170 (see FIG. 9) can beprovided between source 110 and partially reflecting surface 150 toselect a polarization of interest. The polarizer may be tunable ratherthan comprising a set of different polarizers.

As shown in FIG. 7, the target T is placed with substrate W normal tothe optical axis O of objective lens 160. Thus, radiation from source110 is reflected by partially reflecting surface 150 and focused into anillumination spot S (see FIG. 8) on target T on substrate W viaobjective lens 160. In an embodiment, objective lens 160 has a highnumerical aperture (NA), desirably at least 0.9 or at least 0.95. Animmersion metrology apparatus (using a relatively high refractive indexfluid such as water) may even have a numerical aperture over 1.

Rays of illumination 170, 172 focused to the illumination spot fromangles off the axis O gives rise to diffracted rays 174, 176. It shouldbe remembered that these rays are just one of many parallel rayscovering an area of the substrate including target T. Each elementwithin the illumination spot is within the field of view of themetrology apparatus. Since the aperture in plate 130 has a finite width(necessary to admit a useful quantity of radiation), the incident rays170, 172 will in fact occupy a range of angles, and the diffracted rays174, 176 will be spread out somewhat. According to the point spreadfunction of a small target, each diffraction order will be furtherspread over a range of angles, not a single ideal ray as shown.

At least the 0^(th) order diffracted by the target on substrate W iscollected by objective lens 160 and directed back through partiallyreflecting surface 150. An optical element 180 provides at least part ofthe diffracted beams to optical system 182 which forms a diffractionspectrum (pupil plane image) of the target T on sensor 190 (e.g. a CCDor CMOS sensor) using the zeroth and/or first order diffractive beams.In an embodiment, an aperture 186 is provided to filter out certaindiffraction orders so that a particular diffraction order is provided tothe sensor 190. In an embodiment, the aperture 186 allows substantiallyor primarily only zeroth order radiation to reach the sensor 190. In anembodiment, the sensor 190 may be a two-dimensional detector so that atwo-dimensional angular scatter spectrum of a substrate target T can bemeasured. The sensor 190 may be, for example, an array of CCD or CMOSsensors, and may use an integration time of, for example, 40milliseconds per frame. The sensor 190 may be used to measure theintensity of redirected radiation at a single wavelength (or narrowwavelength range), the intensity separately at multiple wavelengths orintegrated over a wavelength range. Furthermore, the sensor may be usedto separately measure the intensity of radiation with transversemagnetic- and/or transverse electric-polarization and/or the phasedifference between transverse magnetic- and transverseelectric-polarized radiation.

Optionally, optical element 180 provides at least part of the diffractedbeams to measurement branch 200 to form an image of the target on thesubstrate W on a sensor 230 (e.g. a CCD or CMOS sensor). The measurementbranch 200 can be used for various auxiliary functions such as focusingthe metrology apparatus (i.e., enabling the substrate W to be in focuswith the objective 160), and/or for dark field imaging of the typementioned in the introduction.

In order to provide a customized field of view for different sizes andshapes of grating, an adjustable field stop 300 is provided within thelens system 140 on the path from source 110 to the objective lens 160.The field stop 300 contains an aperture 302 and is located in a planeconjugate with the plane of the target T, so that the illumination spotbecomes an image of the aperture 302. The image may be scaled accordingto a magnification factor, or the aperture and illumination spot may bein 1:1 size relation. In order to make the illumination adaptable todifferent types of measurement, the aperture plate 300 may comprise anumber of aperture patterns formed around a disc, which rotates to bringa desired pattern into place. Alternatively or in addition, a set ofplates 300 could be provided and swapped, to achieve the same effect.Additionally or alternatively, a programmable aperture device such as adeformable mirror array or transmissive spatial light modulator can beused also.

Typically, a target will be aligned with its periodic structure featuresrunning either parallel to the Y axis or parallel to the X axis. Withregard to its diffractive behavior, a periodic structure with featuresextending in a direction parallel to the Y axis has periodicity in the Xdirection, while the a periodic structure with features extending in adirection parallel to the X axis has periodicity in the Y direction. Inorder to measure the performance in both directions, both types offeatures are generally provided. While for simplicity there will bereference to lines and spaces, the periodic structure need not be formedof lines and space. Moreover, each line and/or space between lines maybe a structure formed of smaller sub-structures. Further, the periodicstructure may be formed with periodicity in two dimensions at once, forexample where the periodic structure comprises posts and/or via holes.

FIG. 8 illustrates a plan view of a typical target T, and the extent ofillumination spot S in the apparatus of FIG. 7. To obtain a diffractionspectrum that is free of interference from surrounding structures, thetarget T, in an embodiment, is a periodic structure (e.g., grating)larger than the width (e.g., diameter) of the illumination spot S. Thewidth of spot S may be smaller than the width and length of the target.The target in other words is ‘underfilled’ by the illumination, and thediffraction signal is essentially free from any signals from productfeatures and the like outside the target itself. This simplifiesmathematical reconstruction of the target as it can be regarded asinfinite. In other embodiments, as described below, the target may notbe fully underfilled and/or misalignment of the radiation spot relativeto the target may cause features outside of the target to contribute tothe signal.

FIG. 9 schematically depicts an example process of the determination ofthe value of one or more variables of interest of a target pattern 30′based on measurement data obtained using metrology. Radiation detectedby the detector 190 provides a measured radiation distribution 108 fortarget 30′.

For the given target 30′, a radiation distribution 208 can becomputed/simulated from a parameterized mathematical model 206 using,for example, a numerical Maxwell solver 210. The parameterizedmathematical model 206 shows example layers of various materials makingup, and associated with, the target. The parameterized mathematicalmodel 206 may include one or more of variables for the features andlayers of the portion of the target under consideration, which may bevaried and derived. As shown in FIG. 9, the one or more of the variablesmay include the thickness t of one or more layers, a width w (e.g., CD)of one or more features, a height h of one or more features, a sidewallangle α of one or more features, and/or relative position betweenfeatures (herein considered overlay). Although not shown, the one ormore of the variables may further include, but is not limited to, therefractive index (e.g., a real or complex refractive index, refractiveindex tensor, etc.) of one or more of the layers, the extinctioncoefficient of one or more layers, the absorption of one or more layers,resist loss during development, a footing of one or more features,and/or line edge roughness of one or more features. One or more valuesof one or more parameters of a 1-D periodic structure or a 2-D periodicstructure, such as a value of width, length, shape or a 3-D profilecharacteristic, may be input to the reconstruction process fromknowledge of the patterning process and/or other measurement processes.For example, the initial values of the variables may be those expectedvalues of one or more parameters, such as a value of CD, pitch, etc.,for the target being measured.

In some cases, a target can be divided into a plurality of instances ofa unit cell. To help ease computation of the radiation distribution of atarget in that case, the model 206 can be designed to compute/simulateusing the unit cell of the structure of the target, where the unit cellis repeated as instances across the full target. Thus, the model 206 cancompute using one unit cell and copy the results to fit a whole targetusing appropriate boundary conditions in order to determine theradiation distribution of the target.

Additionally or alternatively to computing the radiation distribution208 at the time of reconstruction, a plurality of radiationdistributions 208 can be pre-computed for a plurality of variations ofvariables of the target portion under consideration to create a libraryof radiation distributions for use at the time of reconstruction.

The measured radiation distribution 108 is then compared at 212 to thecomputed radiation distribution 208 (e.g., computed near that time orobtained from a library) to determine the difference between the two. Ifthere is a difference, the values of one or more of the variables of theparameterized mathematical model 206 may be varied, a new computedradiation distribution 208 obtained (e.g., calculated or obtained from alibrary) and compared against the measured radiation distribution 108until there is sufficient match between the measured radiationdistribution 108 and the radiation distribution 208. At that point, thevalues of the variables of the parameterized mathematical model 206provide a good or best match of the geometry of the actual target 30′.In an embodiment, there is sufficient match when a difference betweenthe measured radiation distribution 108 and the computed radiationdistribution 208 is within a tolerance threshold.

In these metrology apparatuses, a substrate support may be provided tohold the substrate W during measurement operations. The substratesupport may be similar or identical in form to the substrate table WT ofFIG. 1. In an example where the metrology apparatus is integrated withthe lithographic apparatus, it may even be the same substrate table.Coarse and fine positioners may be provided to accurately position thesubstrate in relation to a measurement optical system. Various sensorsand actuators are provided for example to acquire the position of atarget of interest, and to bring it into position under the objectivelens. Typically many measurements will be made on targets at differentlocations across the substrate W. The substrate support can be moved inX and Y directions to acquire different targets, and in the Z directionto obtain a desired location of the target relative to the focus of theoptical system. It is convenient to think and describe operations as ifthe objective lens is being brought to different locations relative tothe substrate, when, for example, in practice the optical system mayremain substantially stationary (typically in the X and Y directions,but perhaps also in the Z direction) and only the substrate moves.Provided the relative position of the substrate and the optical systemis correct, it does not matter in principle which one of those is movingin the real world, or if both are moving, or a combination of a part ofthe optical system is moving (e.g., in the Z and/or tilt direction) withthe remainder of the optical system being stationary and the substrateis moving (e.g., in the X and Y directions, but also optionally in the Zand/or tilt direction).

In an embodiment, the measurement accuracy and/or sensitivity of atarget may vary with respect to one or more attributes of the beam ofradiation provided onto the target, for example, the wavelength of theradiation beam, the polarization of the radiation beam, the intensitydistribution (i.e., angular or spatial intensity distribution) of theradiation beam, etc. Thus, a particular measurement strategy can beselected that desirably obtains, e.g., good measurement accuracy and/orsensitivity of the target.

In order to monitor the patterning process (e.g., a device manufacturingprocess) that includes at least one pattern transfer step (e.g., anoptical lithography step), the patterned substrate is inspected and oneor more parameters of the patterned substrate are measured/determined.The one or more parameters may include, for example, overlay betweensuccessive layers formed in or on the patterned substrate, criticaldimension (CD) (e.g., critical linewidth) of, for example, featuresformed in or on the patterned substrate, focus or focus error of anoptical lithography step, dose or dose error of an optical lithographystep, optical aberrations of an optical lithography step, placementerror (e.g., edge placement error), etc. This measurement may beperformed on a target of the product substrate itself and/or on adedicated metrology target provided on the substrate. The measurementcan be performed after-development of a resist but before etching or canbe performed after-etch.

There are various techniques for making measurements of the structuresformed in the patterning process, including the use of a scanningelectron microscope, an image-based measurement tool and/or variousspecialized tools. As discussed above, a fast and non-invasive form ofspecialized metrology tool is one in which a beam of radiation isdirected onto a target on the surface of the substrate and properties ofthe scattered (diffracted/reflected) beam are measured. By evaluatingone or more properties of the radiation scattered by the substrate, oneor more properties of the substrate can be determined. This may betermed diffraction-based metrology. One such application of thisdiffraction-based metrology is in the measurement of feature asymmetrywithin a target. This can be used as a measure of overlay, for example,but other applications are also known. For example, asymmetry can bemeasured by comparing opposite parts of the diffraction spectrum (forexample, comparing the −1st and +1^(st) orders in the diffractionspectrum of a periodic grating). This can be done as described above andas described, for example, in U.S. patent application publicationUS2006-066855, which is incorporated herein in its entirety byreference. Another application of diffraction-based metrology is in themeasurement of feature width (CD) within a target. Such techniques canuse the apparatus and methods described above in respect of FIGS. 6-9.

Now, while these techniques are effective, it is desirable to provide analternative measurement technique that derives feature asymmetry withina target (such as overlay, CD asymmetry, sidewall angle asymmetry,etc.). This technique can be effective for specially designed metrologytargets or perhaps more significantly, for determining feature asymmetrydirectly on a device pattern.

Referring to FIG. 10, principles of this measurement technique aredescribed in the context of an overlay embodiment. In FIG. 10A, ageometrically symmetric unit cell of a target T is shown. The target Tcan comprise just a single physical instance of a unit cell or cancomprise a plurality of physical instances of the unit cell as shown inFIG. 10C.

The target T can be a specially designed target. In an embodiment, thetarget is for a scribe lane. In an embodiment, the target can be anin-die target, i.e., the target is among the device pattern (and thusbetween the scribe lanes). In an embodiment, the target can have afeature width or pitch comparable to device pattern features. Forexample, the target feature width or pitches can be less than or equalto 300% of the smallest feature size or pitch of the device pattern, beless than or equal to 200% of the smallest feature size or pitch of thedevice pattern, be less than or equal to 150% of the smallest featuresize or pitch of the device pattern, or be less than or equal to 100% ofthe smallest feature size or pitch of the device pattern.

The target T can be a device structure. For example, the target T can bea portion of a memory device (which often has one or more structuresthat are, or can be, geometrically symmetric as discussed furtherbelow).

In an embodiment, the target T or a physical instance of the unit cellcan have an area of less than or equal to 2400 square microns, an areaof less than or equal to 2000 square microns, an area of less than orequal to 1500 square microns, an area of less than or equal to 1000square microns, an area of less than or equal to 400 square microns,less than or equal to 200 square microns, less than or equal to 100square microns, less than or equal to 50 square microns, less than orequal to 25 square microns, less than or equal to 10 square microns,less than or equal to 5 square microns, less than or equal to 1 squaremicron, less than or equal to 0.5 square microns, or less than or equalto 0.1 square microns. In an embodiment, the target T or a physicalinstance of the unit cell has a cross-sectional dimension parallel tothe plane of the substrate of less than or equal to 50 microns, lessthan or equal to 30 microns, less than or equal to 20 microns, less thanor equal to 15 microns, less than or equal to 10 microns, less than orequal to 5 microns, less than or equal to 3 microns, less than or equalto 1 micron, less than or equal to 0.5 microns, less than or equal to0.2 microns, or less than or equal to 0.1 microns.

In an embodiment, the target T or a physical instance of the unit cellhas a pitch of structures of less than or equal to less than or equal to5 microns, less than or equal to 2 microns, less than or equal to 1micron, less than or equal to 500 nm, less than or equal to 400 nm, lessthan or equal to 300 nm, less than or equal to 200 nm, less than orequal to 150 nm, less than or equal to 100 nm, less than or equal to 75nm, less than or equal to 50 nm, less than or equal to 32 nm, less thanor equal to 22 nm, less than or equal to 16 nm, less than or equal to 10nm, less than or equal to 7 nm or less than or equal to 5 nm.

In an embodiment, the target T has a plurality of physical instances ofthe unit cell. Thus, a target T could typically have the higherdimensions listed here, while the physical instances of the unit cellwill have the lower dimensions listed here. In an embodiment, the targetT comprises 50,000 or more physical instances of the unit cell, 25,000or more physical instances of the unit cell, 15,000 or more physicalinstances of the unit cell, 10,000 or more physical instances of theunit cell, 5,000 or more physical instances of the unit cell, 1000 ormore physical instances of the unit cell, 500 or more physical instancesof the unit cell, 200 or more physical instances of the unit cell, 100or more physical instances of the unit cell, 50 or more physicalinstances of the unit cell, or 10 or more physical instances of the unitcell.

Desirably, the physical instance of the unit cell or the plurality ofphysical instances of the unit cell collectively fills a beam spot ofthe metrology apparatus. In that case, the measured results compriseessentially only information from the physical instance of the unit cell(or its plurality of instances). In an embodiment, the beam spot has across-sectional width of 50 microns or less, 40 microns or less, 30microns or less, 20 microns or less, 15 microns or less, 10 microns orless, 5 microns or less, or 2 microns or less.

The unit cell in FIG. 10A comprises at least two structures that are, orwill be, physically instantiated on the substrate. A first structure1000 comprises lines and a second structure 1005 comprises an oval-typeshape. Of course, the first and second structures 1000, 1005 can bedifferent structures than depicted.

Further, in this example, there can be a relative shift between thefirst and second structures 1000, 1005 from their expected position dueto their separate transfer onto the substrate so as to have an error inoverlay. In this example, the first structure 1000 is located in ahigher layer on a substrate than the second structure 1005. Thus, in anembodiment, the second structure 1005 can be produced in a first lowerlayer in a first execution of a patterning process and the firststructure 1000 can be produced in a second higher layer than the firstlower layer in a second execution of the patterning process. Now, it isnot necessary that the first and second structures 1000, 1005 be locatedin different layers. For example, in a double patterning process(including, for example, an etching process as part thereof), the firstand second structures 1000, 1005 could be produced in a same layer toform essentially a single pattern but there could still be an “overlay”concern in terms of their relative placement within the same layer. Inthis single layer example, both the first and second structures 1000,1005 could have, for example, the form of lines like shown in FIG. 10Afor the first structure 1000 but the lines of the second structure 1005,already provided on the substrate by a first pattern transfer process,could be interleaved with the lines of the structure 1000 provided in asecond pattern transfer process.

Significantly, the unit cell has, or is capable of having, a geometricsymmetry with respect to an axis or point. For example, the unit cell inFIG. 10A has reflection symmetry with respect to, for example, axis 1010and point/rotational symmetry with respect to, for example, point 1015.Similarly, it can be seen that a physical instance of the unit cell (andthus a combination of physical instances of the unit cell) in FIG. 10Chas a geometric symmetry.

In an embodiment, the unit cell has a geometric symmetry for a certainfeature (such as overlay). Embodiments herein focus on the unit cellhaving zero overlay when it is geometrically symmetric. However,instead, the unit cell can have zero overlay for a certain geometricasymmetry. Appropriate offsets and calculations would then be used toaccount for the unit cell having a zero overlay when it has a certaingeometric asymmetry. Pertinently, the unit cell should be capable ofchange in symmetry (e.g., become asymmetric, or become furtherasymmetric, or become symmetric from an asymmetric situation) dependingon the certain feature value.

In the example of FIG. 10A, the unit cell has a geometric symmetry for azero overlay (although it need not be zero overlay). This is representedby the arrows 1020 and 1025 which shows that the lines of the firststructure 1000 are evenly aligned with respect to the oval-type shape ofthe second structure 1005 (and which even alignment at least in partenables the unit cell to have geometric symmetry as shown in FIG. 10A).So, in this example, when the unit cell has geometric symmetry, there iszero overlay. However, when there is an error in overlay (e.g., anon-zero overlay), the unit cell is no longer geometrically symmetricand by definition the target is no longer geometrically symmetric.

Further, where a target comprises a plurality of physical instances ofthe unit, the instances of the unit cell are arranged periodically. Inan embodiment, the instances of the unit cell are arranged in a lattice.In an embodiment, the periodic arrangement has a geometric symmetrywithin the target.

So, in this technique, as discussed further hereafter, advantage istaken of the change in geometric symmetry (e.g., a change to a geometricasymmetry, or change to a further geometric asymmetry, or a change fromgeometric asymmetry to geometric symmetry) related to a featureasymmetry of interest (e.g., non-zero overlay) to be able to determinethe feature asymmetry (e.g., non-zero overlay).

A target comprising a physical instance of the unit cell of FIG. 10A canbe illuminated with radiation using, for example, the metrologyapparatus of FIG. 7. The radiation redirected by the target can bemeasured, e.g., by detector 190. In an embodiment, a pupil of theredirected radiation is measured, i.e., a Fourier transform plane. Anexample measurement of such a pupil is depicted as pupil image 1030.While the pupil image 1030 has a diamond-type shape, it need not havesuch a shape. The term pupil and pupil plane herein includes anyconjugates thereof unless the context otherwise requires (for example,where a pupil plane of a particular optical system is being identified).The pupil image 1030 is effectively an image, specified in terms of anoptical characteristic (in this case intensity), of a pupil of theredirected radiation.

For convenience, the discussion herein will focus on intensity as anoptical characteristic of interest. But, the techniques herein may beused with one or more alternative or additional optical characteristics,such as phase and/or reflectivity.

Further, for convenience, the discussion herein focuses on detecting andprocessing images of redirected radiation and in particular pupilimages. However, the optical properties of the redirected radiation canbe measured and represented in different manners than images. Forexample, the redirected radiation can be processed in terms of one ormore spectrums (e.g., intensity as a function of wavelength). Thus, adetected image of redirected radiation can be considered as an exampleof an optical representation of the redirected radiation. So, in thecase of a pupil plane image, a pupil image is an example of a pupilrepresentation.

Further, the redirected radiation can be polarized or non-polarized. Inan embodiment, the measurement beam radiation is polarized radiation. Inan embodiment, the measurement beam radiation is linearly polarized.

In an embodiment, a pupil representation is of primarily, orsubstantially, one diffraction order of redirected radiation from thetarget. For example, the radiation can be 80% or more, 85% or more, 90%or more, 95% or more, 98% or more or 99% or more, of a particular orderof the radiation. In an embodiment, the pupil representation is ofprimarily, or substantially, zeroth order redirected radiation. This canoccur, for example, when the pitch of the target, the wavelength of themeasurement radiation, and optionally one or more other conditions causethe target to redirect primarily zeroth order (although there can beradiation of one or more higher orders). In an embodiment, a majority ofthe pupil representation is zeroth order redirected radiation. In anembodiment, the pupil representation is of zeroth radiation andseparately of 1^(st) order radiation, which can then be linearlycombined (superposition). The aperture 186 in FIG. 7 can be used toselect a particular order, e.g., the zeroth order, of radiation.

Having regard to pupil image 1030 corresponding to the geometricallysymmetric unit cell of the first and second structures 1000, 1005, itcan be seen that the intensity distribution is essentially symmetricwithin the pupil image (e.g., with the same symmetry type as of thegeometric structure). This is further confirmed by removing thesymmetric intensity distribution portion from the pupil image 1030,which results in the derived pupil image 1035. To remove the symmetricintensity distribution portion, a particular pupil image pixel (e.g., apixel) can have the symmetric intensity distribution portion removed bysubtracting from the intensity at that particular pupil image pixel theintensity of a symmetrically located pupil image pixel, and vice versa.In an embodiment, the pixel can correspond to the pixels of the detector(e.g., detector 190), but it need not; for example, a pupil image pixelcould be a plurality of the pixels of the detector. In an embodiment,the point or axis of symmetry across which pixel intensities aresubtracted corresponds with a point or axis of symmetry of the unitcell. So, for example, considering pupil image 1030, the symmetryintensity distribution portion can be removed by, for example,subtracting from the intensity I_(i) at that particular pixel shown theintensity I_(i)′ from a symmetrically located pixel, i.e., symmetricallylocated with respect to axis 1032. Thus, the intensity at a particularpixel with the symmetrical intensity portion removed, S_(i), is thenS_(i)=I_(i)−I_(i)′. This can be repeated for a plurality of pixels ofthe pupil image, e.g., all the pixels in the pupil image. As seen in thederived pupil image 1035, the intensity distribution corresponding tothe symmetric unit cell is essentially completely symmetric. Thus, asymmetric target with a symmetric unit cell geometry (and if applicable,a certain periodicity of instances of the unit cell) results in asymmetric pupil response as measured by a metrology apparatus.

Referring now to FIG. 10B, an example of an error in overlay is depictedwith respect to the unit cell depicted in FIG. 10A. In this case, thefirst structure 1000 is shifted in the X-direction with respect to thesecond structure 1005. In particular, the axis 1010 centered on thelines of the first structure 1000 has shifted to the right in FIG. 10Bto axis 1045. Thus, there is an error in the overlay 1040 in theX-direction; that is, an X direction overlay error. Of course, thesecond structure 1005 could be shifted relative to the first structure1000 or both could be shifted relative to each other. In any event, theresult is an X direction overlay error. However, as should beappreciated from this unit cell arrangement, a purely relative shift inthe Y-direction between the first structure 1000 and the secondstructure 1005 would not change the geometric symmetry of this unitcell. But, with an appropriate geometric arrangement, overlay in twodirections or between different combinations of parts of the unit cellcan change symmetry and could also be determined, as further discussedbelow.

As a consequence of the change in the physical configuration of the unitcell from the nominal physical configuration of the unit cell in FIG.10A and represented by the error in overlay 1040, the result is that theunit cell has become geometrically asymmetric. This can be seen by thearrows 1050 and 1055 of different length, which show that the oval-typeshape of the second structure 1005 is unevenly located relative to thelines of the first structure 1000. The symmetry is examined with respectto the point or axis of symmetry of the pupil image 1030, i.e. in thatcase, axis 1032 which is now shown axis 1034.

The physical instance of the unit cell of FIG. 10B can be illuminatedwith radiation using, for example, the metrology apparatus of FIG. 7. Apupil image of the redirected radiation can be recorded, e.g., bydetector 190. An example of such a pupil image is depicted as pupilimage 1060. The pupil image 1060 is effectively an image of theintensity. While the pupil image 1060 has a diamond-type shape, it neednot have such a shape; it can be a circular shape or any other shape.Moreover, the pupil image 1060 is of a substantially same axis orcoordinate location as pupil image 1030. That is, in this embodiment, anaxis of symmetry 1010 in the unit cell of FIG. 10A and the same axis inthe unit cell of FIG. 10B align with an axis of symmetry 1032 of thepupil images 1030, 1060.

Having regard to pupil image 1060 corresponding to the geometricallyasymmetric unit cell of the first and second structures 1000, 1005, itvisually seems like the intensity distribution is essentially symmetricwithin the pupil image. However, there is an asymmetric intensitydistribution portion within the pupil image. This asymmetric intensitydistribution portion is due to the asymmetry in the unit cell. Moreover,the asymmetric intensity distribution is significantly lower inmagnitude than a symmetric intensity distribution portion in the pupilimage.

So, in an embodiment, to more effectively isolate the asymmetricintensity distribution portion, the symmetric intensity distributionportion can be removed from the pupil image 1060, which results in thederived pupil image 1065. Like with obtaining derived pupil image 1035,a particular pupil image pixel (e.g., a pixel) can have the symmetricintensity distribution portion removed by subtracting from the intensityat that particular pupil image pixel the intensity of a symmetricallylocated pupil image pixel, and vice versa, as discussed above. So, forexample, considering pupil image 1060, the symmetry intensitydistribution portion can be removed by, for example, subtracting fromthe intensity I_(i) at that particular pixel shown the intensity I_(i)′from a symmetrically located pixel, i.e., symmetrically located withrespect to axis 1032 to yield S_(i). This can be repeated for aplurality of pixels of the pupil image, e.g., all the pixels in thepupil image. In FIGS. 10A and 10B, the full derived pupil images ofS_(i) are depicted for explanation purposes. As will be appreciated,half of a derived pupil image of FIG. 10A or 10B is the same as theother half thereof. So, in an embodiment, the values from only half ofthe pupil image can be used for further processing discussed herein andso a derived image pupil used in further processing herein can be onlyhalf of the S_(i) values for a pupil.

As seen in the derived pupil image 1065, the intensity distributionmeasured using a physical instance of an asymmetric unit cell is notsymmetric. As seen in regions 1075 and 1080, there is an asymmetricintensity distribution portion visible once the symmetric intensitydistribution portion is removed. As noted above, the full derived pupilimage 1065 is shown and so the asymmetric intensity distribution portionis shown on both halves (even though they are equal to each other interms of magnitude and distribution in their respective halves).

Thus, an asymmetry in the geometrical domain corresponds to an asymmetryin the pupil. So, in an embodiment, a method is provided that uses theoptical response of a periodic target that possesses, or is capable of,inherent geometric symmetry in its physical instance of a unit cell todetermine a parameter corresponding to a physical configuration changethat causes a change in geometric symmetry (e.g., cause an asymmetry, orcause a further asymmetry, or cause an asymmetric unit cell to becomesymmetric) of the physical instance of the unit cell. In particular, inan embodiment, an overlay induced asymmetry (or lack thereof) in thepupil as measured by a metrology apparatus can be exploited to determinethe overlay. That is, the pupil asymmetry is used to measure the overlaywithin the physical instance of the unit cell and thus within thetarget.

The change in symmetry in the geometrical domain of the target T canarise due to a relative shift between first and second structures 1000,1005 from their expected positioning. The relative shift may occur dueto an error in overlay between a patterning process used to form thefirst structure 1000 and a patterning process used to form the secondstructure 1005.

Further details of techniques described herein can be found in U.S.Patent Application Publication No. US 2017/0255736, which isincorporated herein in its entirety.

FIG. 11 depicts example targets T (depicted as circular features)positioned in scribe lanes 72 surrounding device regions 70. The deviceregions 70 are regions configured to comprise device structurescorresponding to products to be manufactured. Positioning targets T inscribe lanes 72 allows targets T to be relatively large. Where it isdesired to perform metrology measurements at a higher spatial densityover the substrate W, it may be necessary to position targets T atlocations other than in the scribe lanes 72. This may be desirable, forexample, where higher order corrections are to be implemented usingmeasurements of the targets T. So, it may be desirable, for example, toposition targets within the device regions 70. The space available forindividual targets T at locations other than in the scribe lanes 72 mayrequire targets T to be very small. Targets T may be smaller than 10×10μm², optionally about 5×5 μm², for example.

For small targets T (e.g. 5×5 μm² targets), alignment of a radiationspot of a metrology process with the target T may be achieved byperforming pattern recognition on an image of the target T observed bythe metrology apparatus. This may be done by first performing arelatively large jump towards the general area of the target T (based onthe position of a larger target for example). A pattern recognitionprocess can be used to identify the precise location of the target T. Asmaller jump is then performed to align the radiation spot as closely aspossible on the target T.

It has proved challenging to achieve alignment with desired accuracy forsmall targets. Misalignment between a radiation spot and a target T cancause a region outside of the target T to contribute to a greater extentto a signal measured by the metrology process, thereby introducingerrors.

A pattern recognition process may not always reliably distinguish thetarget T from the region around the target (its surroundings). Thepattern recognition process may need to take account of properties ofthe region around the target T when alignment is performed. This meansthat the pattern recognition process is position dependent and multiplepattern recognition recipes may be needed for dealing with measurementof a whole substrate. This requirement significantly complicates theoverall metrology process. The sequence of large jump—patternrecognition—smaller jump can be time-consuming.

Variations in focus define the size of the radiation spot and may alsoaffect the extent to which the region outside of the target contributesto the signal.

Embodiments of the present disclosure aim to at least partially addressone or more issues related to alignment and/or focus.

FIG. 12 schematically depicts a method of obtaining measurement data 310for optimizing a metrology process. Start and end points of the methodare marked S and E respectively.

In step S1, a metrology process is applied to a first target T on asubstrate W. The metrology process comprises illuminating the firsttarget T with a radiation spot and detecting radiation redirected by thefirst target T. The method comprises a plurality of applications of themetrology process to the first target T. Thus, the first target T ismeasured plural times. In some embodiments, the applications of themetrology process include applications at plural different positions(corresponding to different alignments) of the radiation spot relativeto the first target T. In some embodiments, the applications of themetrology process include applications at plural different focus heightsof the radiation spot. In some embodiments, the applications of themetrology process include applications at plural different positions andplural different focus heights.

The plural applications of the metrology process may be achieved via acontrol loop in which a decision step S2 is used to determine whether ornot a planned sequence of applications of the metrology process has beencompleted. The decision step S2 causes the method to loop through ametrology process settings adjustment step S3 and the application of themetrology process in step S1 until all required applications of themetrology process in step S1 have been completed. The metrology processsettings adjustment step S3 may comprise adjusting the metrology processso that the nominal alignment and/or focus height of the radiation spotis adjusted, for example. The metrology process settings adjustment stepS3 may be further configured to adjust a rotational position of thesubstrate W. In an embodiment, for each of one or more (or all)alignment and/or focus settings, the metrology process is performed attwo angular positions that are 180 degrees apart from each other. Thisapproach reduces the contribution to asymmetry in the detected pupilrepresentation from asymmetry in the metrology apparatus (e.g. in theoptics and/or sensors). Such asymmetry may be referred to as sensorasymmetry. When all required applications of the metrology process instep S1 have been completed the decision step S2 directs the method tostep S4. Step S4 comprises outputting the measurement data 310. Themeasurement data 310 output in step S4 may be stored or transmitteddirectly to one or more other data processing apparatuses.

In each application of the metrology process in step S1 the radiationmay be directed onto the target T and the redirected radiation detectedas described above with reference to FIGS. 7-10C. The detectedredirected radiation may comprise a pupil representation of an opticalcharacteristic of radiation in a pupil plane, as described above withreference to FIGS. 7-10C. Thus, the measurement data 310 may comprise,for each application of the metrology process in step S1, a detectedpupil representation of an optical characteristic of the redirectedradiation in a pupil plane. The measurement data 310 may thus comprise aplurality of detected pupil representations obtained at differentnominal alignments and/or different nominal focus heights. The opticalcharacteristic may comprise a radiation intensity or phase. In the casewhere the optical characteristic comprises a radiation intensity, thedetected pupil representation may be referred to as a pupil image. Ametrology apparatus of the type described with reference to FIG. 7 maybe used for example to perform the metrology process. The detected pupilrepresentation of radiation may comprise primarily zeroth orderradiation, as described above. This may be particularly desirable wherethe target T comprises a high resolution target, such as a devicestructure. Thus, in an embodiment, the target T comprises a devicestructure. In other embodiments, the target T comprises a non-devicestructure within a device region 70 (see FIG. 11) configured tocomprise, or comprising, device structures.

FIG. 13 schematically depicts a method of optimizing a metrologyprocess. Start and end points of the method are marked S and Erespectively. In step S11, the method comprises obtaining measurementdata 310 and a reference pupil representation 312. The measurement data310 is derived from a plurality of applications of the metrology processto a target T. In an embodiment, the plurality of applications of themetrology process are performed using a sensor in video mode. In otherembodiments, the plurality of applications of the metrology processcomprise individual discrete measurements (i.e. not in video mode). Inan embodiment, the measurement data 310 is generated using the methoddescribed above with reference to FIG. 12. The measurement data 310 maythus comprise, for each application of the metrology process, a detectedpupil representation of an optical characteristic of redirectedradiation in a pupil plane. Details about how the reference pupilrepresentation 312 may be generated or obtained are given later.

In step S12, an optimal alignment, an optimal focus height, or both anoptimal alignment and an optimal focus height are determined. Thealignment and/or focus height are optimized in the sense that steps aretaken to tune these parameters to improve a performance of the metrologyprocess relative to where the optimization is not applied. The termoptimal does not require an absolute optimum, in the sense that nofurther improvement could possibly be obtained.

In embodiments, the alignment and/or focus height are optimized in stepS12 by comparing each of the detected pupil representations in themeasurement data 310 with the reference pupil representation 312. Thecomparison may aim for example to identify which of the detected pupilrepresentations is most similar to the reference pupil representation312. Similarity with the reference pupil representation 312 may beassociated with closeness to an optimal alignment and/or optimal focus.In an embodiment, an alignment associated with the detected pupilrepresentation determined to be the most similar to the reference pupilrepresentation is determined to be the optimal alignment. In anembodiment, a focus height associated with the detected pupilrepresentation determined to be the most similar to the reference pupilrepresentation is determined to be the optimal focus height.

In an embodiment, the comparisons between the detected pupilrepresentations and the reference pupil representation comprisecalculating a degree of similarity between each detected pupilrepresentation and the reference pupil representation. Variousmathematical techniques are known for determining degrees of similaritybetween different entities. Similarity is sometimes referred to ascorrelation. In an embodiment, a correlation response describing a crosscorrelation between each detected pupil representation and the referencepupil representation is used to provide a quantitative measure of thedegree of similarity. In some embodiments, a Pearson's correlation orRsquared of a regression technique may be used to represent the degreeof similarity.

FIG. 14 depicts an example set of detected pupil representations 320obtained for plural different alignments and focus heights. In thisexample, alignment varies along each row, as indicated schematically byarrow 322. Thus, each detected pupil representation 320 in any given rowrepresents a detected pupil representation obtained by the metrologyprocess for a different alignment of the radiation spot relative to thefirst target T being measured. The different alignments may, forexample, represent different positions along a X direction, along a Ydirection, or both, where the X and Y directions correspond toorthogonal directions in a XY plane parallel to the plane of thesubstrate W on which the first target T is formed. In this example,focus height varies along each column, as indicated schematically byarrow 324. Thus, each row corresponds to a set of measurements performedat the same nominal focus height but different alignments.

FIG. 15 schematically depicts variation of a parameter Corr representinga degree of similarity (e.g. a cross correlation) between a detectedpupil representation and a reference pupil representation as a functionof alignment and/or focus (Pos/F). The variation is shown inone-dimension for clarity, but it will be appreciated that the variationmay in general be multivariate, depending for example on a position inthe X direction, a position in the Y direction, and a focus height(which may be considered as a position along a Z direction perpendicularto the X and Y directions). The determining of the optimal alignmentand/or optimal focus height may comprise determining where thesimilarity or correlation is maximum. Any suitable optimizationprocedure may be used to locate the maximum. In the example shown, themaximum similarity or correlation occurs at the position MAX, which maytherefore be taken as the optimal alignment and/or optimal focus height.The value of Corr at the point MAX need not be 1 (indicating perfectmatching between the detected pupil representation and the referencepupil representation).

The reference pupil representation may be generated in various ways.

In some embodiments, the reference pupil representation is generatedusing a simulation of redirection of radiation from the first target T.The simulated pupil representation may be referred to as a syntheticpupil representation or synthetic pupil image. In some embodiments, thesimulation is based on a parametrized mathematical model 206 using, forexample, a numerical Maxwell solver 210, as described above withreference to FIG. 9. The simulation may thus be based on a mathematicalmodel of the first target T. The simulation may be based on perfectalignment and/or focus height of the radiation spot relative to thefirst target T.

In some embodiments, the reference pupil representation is generatedusing a detected pupil representation obtained by a previous applicationof the metrology process. The previous application of the metrologyprocess may have been performed at an optimal alignment and/or optimalfocus height (as determined using methods according to embodiments ofthe present disclosure or using other methods).

In some embodiments, the previous application of the metrology processcomprises an application of the metrology process to a second target,the second target being larger than the first target. The effect ofmisalignment of the radiation spot relative to the center of a target Tbecomes larger as the target size is reduced, due to the increasedinfluence from regions outside of the target for small targets relativeto larger targets. The application of the metrology process to thelarger second target T is therefore likely to produce a detected pupilrepresentation which is closer to the detected pupil representationexpected for perfect alignment and/or perfect focus height, for a givenlevel of misalignment and/or focus error, than would be achieved byapplication of the metrology process to the smaller first target T. Byusing the second target to provide the reference pupil representationand selecting a detected pupil representation from the first target thatis most similar to the reference pupil representation, embodiments ofthe present disclosure at least partially overcome some of thechallenges associated with performing metrology on small targets.

In an embodiment, the first target T and the second target T eachcomprise a periodic structure defined by a unit cell that has the sameperiodicity in at least one direction. In some embodiments, the unitcell of the second target T is identical to the unit cell of the firsttarget T, for example having the same sequence of layers and the sameperiodicities in all directions. The first target and second target maythus differ only in the number of unit cells that are present in thetarget T.

In an embodiment, the substrate W comprises one or more device regions70 (as depicted in FIG. 11) configured to comprise device structures andone or more scribe lanes 72 positioned outside of the device regions 70.In such embodiments, the first target T may be located in one of thedevice regions (not shown) and the second target T may be provided inone of the scribe lanes 72 (where there is room to accommodate thelarger target). In an embodiment, the first target T is equal to orsmaller than about 5×5 μm² and the second target T is equal to or largerthan about 8×8 μm² (e.g. about 10×10 μm²). In an embodiment, the deviceregions 70 are provided in a plurality of fields, with each field havingone or more second targets T associated with that field. In suchembodiments, for each of two or more of the fields, one or more secondtargets T corresponding to the field are used to generate the referencepupil representation for determining the optimal alignment and/oroptimal focus height for one or more (or all) of the first targets Tthat are measured in that field. By using local second targets T toprovide the reference pupil representation for each field and updatingthe reference pupil representation from field to field, errorsassociated with process variations across the substrate W can bereduced.

In some embodiments, the reference pupil representation is generatedusing machine learning based on plural previous applications of themetrology process with different alignments and focus heights.

The novel approach of determining optimal alignment and/or focus heightsbased on comparisons between detected pupil representations and areference pupil representation 312 makes it possible reliably to achieveaccurate alignment and/or focus of a radiation spot without having toobtain and/or analyze an image of the radiation spot on the firsttarget, for example using complex pattern recognition techniques. Theapproach may therefore improve throughput and/or avoid data processingrequirements, time requirements and/or performance shortcomingsassociated with pattern recognition. The approach allows alignmentand/or focus to be set more accurately than prior approaches, therebyreducing the negative influence on signal quality from regions outsideof small targets. The approach also makes it possible to determinealignment and/or focus using the same sensor and/or radiationwavelengths that are used to obtain values of parameters of interest(e.g. overlay, CD, etc.), which may also contribute to improved accuracyand/or throughput. It is no longer necessary for example to have adedicated measurement and subsequent pattern recognition procedure todetermine alignment. Furthermore, a unified processing of theinformation from the pupil plane is made possible. This means, forexample, that the alignment and/or focus height optimization could beperformed using the same settings (e.g. radiation spot parameters,polarization, wavelength, etc.) as for an inference to obtain values ofa parameter of interest (e.g. overlay, CD). Thus, the identified optimalalignment and/or optimal focus height can be such as to minimize errorsin the inference, thereby reducing errors in values of a parameter ofinterest obtained based on the determined optimal alignment and/oroptimal focus height. Detected pupil representations obtained atalignments and/or focus heights that are known to be away from theoptimal alignment and/or optimal focus height may be used in theinference to provide information about structures located in the regionoutside of the first target T, thereby further improving the inference.

In an embodiment, the measurements at different positions and/ordifferent focus heights are used to quantify a sensitivity to alignmentand/or focus height errors. This quantification of sensitivity may beperformed for example by quantifying how quickly the degree ofsimilarity Corr (see FIG. 15) between the detected pupil representationsand the reference pupil representation varies moving away from theoptimal alignment and/or optimal focus height. When visualized as shownin FIG. 15, the sensitivity may be related to the curvature of Corr atthe maximum MAX (which may be quantified for example via the secondderivative of Corr with respect to Pos/F).

In an embodiment, the optimal alignment and/or optimal focus heightdetermined in step S12 is output as settings data 314. The settings data314 may be used to control a subsequent metrology process to operatenominally at the optimal alignment and/or optimal focus height. A valueof a parameter of interest (e.g. overlay, CD) may thus be obtained morereliably and/or accurately in the subsequent metrology process. Thesettings data 314 may be used to provide a control signal that controlsone or more operational parameters of a metrology apparatus relevant toalignment and/or focus height. The control signal may be fed, forexample, to the processor PU in the metrology apparatus of FIG. 7.

In other embodiments, the detected pupil representation that correspondsto the optimal alignment and/or optimal focus height determined in stepS12 may itself be analyzed in step S13 to obtain a value of a parameterof interest (e.g. overlay, CD). Thus, the same measurement data is usedboth for determining an optimal alignment and/or optimal focus heightand for calculating a value of a parameter of interest based on thedetermined optimal alignment and/or optimal focus height. The obtainedvalue of a parameter of interest may be output as output data 316.

In an embodiment, the method is applied to each of a plurality of firsttargets T at different locations on the substrate T. The plurality ofpositions of the radiation spot used when applying the method to atleast one of the first targets is then selected using an optimalalignment obtained by applying the method to at least one other of thefirst targets. In an embodiment, the plurality of positions are selectedso as to be proximate (e.g. centered on) the optimal alignment obtainedby applying the method to the at least one other first target T. Thisapproach can thus make use of previous applications of the method tostart the optimization process nearer to the optimal alignment. Therange of alignments to consider in the optimization process can thus bereduced, thereby improving speed and/or allowing more time to be spentmeasuring at alignments that are likely to be close to optimalalignment.

Referring to FIG. 16, a computer system 3200 is shown. The computersystem 3200 includes a bus 3202 or other communication mechanism forcommunicating information, and a processor 3204 (or multiple processors3204 and 3205) coupled with bus 3202 for processing information.Computer system 3200 also includes a main memory 3206, such as a randomaccess memory (RAM) or other dynamic storage device, coupled to bus 3202for storing information and instructions to be executed by processor3204. Main memory 3206 also may be used for storing temporary variablesor other intermediate information during execution of instructions to beexecuted by processor 3204. Computer system 3200 further includes a readonly memory (ROM) 3208 or other static storage device coupled to bus3202 for storing static information and instructions for processor 3204.A storage device 3210, such as a magnetic disk or optical disk, isprovided and coupled to bus 3202 for storing information andinstructions.

Computer system 3200 may be coupled via bus 3202 to a display 3212, suchas a cathode ray tube (CRT) or flat panel or touch panel display fordisplaying information to a computer user. An input device 3214,including alphanumeric and other keys, is coupled to bus 3202 forcommunicating information and command selections to processor 3204.Another type of user input device is cursor control 3216, such as amouse, a trackball, or cursor direction keys for communicating directioninformation and command selections to processor 3204 and for controllingcursor movement on display 3212. This input device typically has twodegrees of freedom in two axes, a first axis (e.g., x) and a second axis(e.g., y), that allows the device to specify positions in a plane. Atouch panel (screen) display may also be used as an input device.

The computer system 3200 may be suitable to function as a processingunit herein in response to processor 3204 executing one or moresequences of one or more instructions contained in main memory 3206.Such instructions may be read into main memory 3206 from anothercomputer-readable medium, such as storage device 3210. Execution of thesequences of instructions contained in main memory 3206 causes processor3204 to perform a process described herein. One or more processors in amulti-processing arrangement may also be employed to execute thesequences of instructions contained in main memory 3206. In alternativeembodiments, hard-wired circuitry may be used in place of or incombination with software instructions. Thus, embodiments are notlimited to any specific combination of hardware circuitry and software.

The term “computer-readable medium” as used herein refers to any mediumthat participates in providing instructions to processor 3204 forexecution. Such a medium may take many forms, including but not limitedto, non-volatile media, volatile media, and transmission media.Non-volatile media include, for example, optical or magnetic disks, suchas storage device 3210. Volatile media include dynamic memory, such asmain memory 3206.

Transmission media include coaxial cables, copper wire and fiber optics,including the wires that comprise bus 3202. Transmission media can alsotake the form of acoustic or light waves, such as those generated duringradio frequency (RF) and infrared (IR) data communications. Common formsof computer-readable media include, for example, a floppy disk, aflexible disk, hard disk, magnetic tape, any other magnetic medium, aCD-ROM, DVD, any other optical medium, punch cards, paper tape, anyother physical medium with patterns of holes, a RAM, a PROM, and EPROM,a FLASH-EPROM, any other memory chip or cartridge, a carrier wave asdescribed hereinafter, or any other medium from which a computer canread.

Various forms of computer readable media may be involved in carrying oneor more sequences of one or more instructions to processor 3204 forexecution. For example, the instructions may initially be borne on amagnetic disk of a remote computer. The remote computer can load theinstructions into its dynamic memory and send the instructions over atelephone line using a modem. A modem local to computer system 3200 canreceive the data on the telephone line and use an infrared transmitterto convert the data to an infrared signal. An infrared detector coupledto bus 3202 can receive the data carried in the infrared signal andplace the data on bus 3202. Bus 3202 carries the data to main memory3206, from which processor 3204 retrieves and executes the instructions.The instructions received by main memory 3206 may optionally be storedon storage device 3210 either before or after execution by processor3204.

Computer system 3200 may also include a communication interface 3218coupled to bus 3202. Communication interface 3218 provides a two-waydata communication coupling to a network link 3220 that is connected toa local network 3222. For example, communication interface 3218 may bean integrated services digital network (ISDN) card or a modem to providea data communication connection to a corresponding type of telephoneline. As another example, communication interface 3218 may be a localarea network (LAN) card to provide a data communication connection to acompatible LAN. Wireless links may also be implemented. In any suchimplementation, communication interface 3218 sends and receiveselectrical, electromagnetic or optical signals that carry digital datastreams representing various types of information.

Network link 3220 typically provides data communication through one ormore networks to other data devices. For example, network link 3220 mayprovide a connection through local network 3222 to a host computer 3224or to data equipment operated by an Internet Service Provider (ISP)3226. ISP 3226 in turn provides data communication services through theworldwide packet data communication network, now commonly referred to asthe “Internet” 3228. Local network 3222 and Internet 3228 both useelectrical, electromagnetic or optical signals that carry digital datastreams. The signals through the various networks and the signals onnetwork link 3220 and through communication interface 3218, which carrythe digital data to and from computer system 3200, are exemplary formsof carrier waves transporting the information.

Computer system 3200 can send messages and receive data, includingprogram code, through the network(s), network link 3220, andcommunication interface 3218. In the Internet example, a server 3230might transmit a requested code for an application program throughInternet 3228, ISP 3226, local network 3222 and communication interface3218. In accordance with one or more embodiments, one such downloadedapplication provides for a method as disclosed herein, for example. Thereceived code may be executed by processor 3204 as it is received,and/or stored in storage device 3210, or other non-volatile storage forlater execution. In this manner, computer system 3200 may obtainapplication code in the form of a carrier wave.

An embodiment of the disclosure may take the form of a computer programcontaining one or more sequences of machine-readable instructionsdescribing a method as disclosed herein, or a data storage medium (e.g.semiconductor memory, magnetic or optical disk) having such a computerprogram stored therein. Further, the machine readable instruction may beembodied in two or more computer programs. The two or more computerprograms may be stored on one or more different memories and/or datastorage media.

Any controllers described herein may each or in combination be operablewhen the one or more computer programs are read by one or more computerprocessors located within at least one component of the lithographicapparatus. The controllers may each or in combination have any suitableconfiguration for receiving, processing, and sending signals. One ormore processors are configured to communicate with the at least one ofthe controllers. For example, each controller may include one or moreprocessors for executing the computer programs that includemachine-readable instructions for the methods described above. Thecontrollers may include data storage medium for storing such computerprograms, and/or hardware to receive such medium. So the controller(s)may operate according the machine readable instructions of one or morecomputer programs.

Although specific reference may be made in this text to the use of ametrology apparatus in the manufacture of ICs, it should be understoodthat the metrology apparatus and processes described herein may haveother applications, such as the manufacture of integrated opticalsystems, guidance and detection patterns for magnetic domain memories,flat-panel displays, liquid-crystal displays (LCDs), thin film magneticheads, etc. The skilled artisan will appreciate that, in the context ofsuch alternative applications, any use of the terms “wafer” or “die”herein may be considered as synonymous with the more general terms“substrate” or “target portion”, respectively. The substrate referred toherein may be processed, before or after exposure, in for example atrack (a tool that typically applies a layer of resist to a substrateand develops the exposed resist), a metrology tool and/or one or morevarious other tools. Where applicable, the disclosure herein may beapplied to such and other substrate processing tools. Further, thesubstrate may be processed more than once, for example in order tocreate a multi-layer IC, so that the term substrate used herein may alsorefer to a substrate that already contains multiple processed layers.

Although specific reference may have been made above to the use ofembodiments of the disclosure in the context of optical lithography, itwill be appreciated that the disclosure may be used in otherapplications, for example nanoimprint lithography, and where the contextallows, is not limited to optical lithography. In the case ofnanoimprint lithography, the patterning device is an imprint template ormold.

The terms “radiation” and “beam” used herein encompass all types ofelectromagnetic radiation, including ultraviolet (UV) radiation (e.g.having a wavelength of or about 365, 355, 248, 193, 157 or 126 nm) andextreme ultra-violet (EUV) radiation (e.g. having a wavelength in therange of 5-20 nm), as well as particle beams, such as ion beams orelectron beams.

The term “lens”, where the context allows, may refer to any one orcombination of various types of optical components, includingrefractive, reflective, magnetic, electromagnetic and electrostaticoptical components.

References herein to crossing or passing a threshold may includesomething having a value lower than a specific value or lower than orequal to a specific value, something having a value higher than aspecific value or higher than or equal to a specific value, somethingbeing ranked higher or lower than something else (through e.g., sorting)based on, e.g., a parameter, etc.

References herein to correcting or corrections of an error includeeliminating the error or reducing the error to within a tolerance range.

The term “optimizing” and “optimization” as used herein refers to ormeans adjusting a lithographic apparatus, a patterning process, etc.such that results and/or processes of lithography or patterningprocessing have more a desirable characteristic, such as higher accuracyof projection of a design layout on a substrate, a larger processwindow, etc. Thus, the term “optimizing” and “optimization” as usedherein refers to or means a process that identifies one or more valuesfor one or more variables that provide an improvement, e.g. a localoptimum, in at least one relevant metric, compared to an initial set ofone or more values for those one or more variables. “Optimum”, “optimal”and other related terms should be construed accordingly. In anembodiment, optimization steps can be applied iteratively to providefurther improvements in one or more metrics.

In an optimization process of a system, a figure of merit of the systemor process can be represented as a cost function. The optimizationprocess boils down to a process of finding a set of parameters (designvariables) of the system or process that optimizes (e.g., minimizes ormaximizes) the cost function. The cost function can have any suitableform depending on the goal of the optimization. For example, the costfunction can be weighted root mean square (RMS) of deviations of certaincharacteristics (evaluation points) of the system or process withrespect to the intended values (e.g., ideal values) of thesecharacteristics; the cost function can also be the maximum of thesedeviations (i.e., worst deviation). The term “evaluation points” hereinshould be interpreted broadly to include any characteristics of thesystem or process. The design variables of the system can be confined tofinite ranges and/or be interdependent due to practicalities ofimplementations of the system or process. In the case of a lithographicapparatus or patterning process, the constraints are often associatedwith physical properties and characteristics of the hardware such astunable ranges, and/or patterning device manufacturability design rules,and the evaluation points can include physical points on a resist imageon a substrate, as well as non-physical characteristics such as dose andfocus.

While specific embodiments of the disclosure have been described above,it will be appreciated that the disclosure may be practiced otherwisethan as described. For example, the disclosure may take the form of acomputer program containing one or more sequences of machine-readableinstructions describing a method as disclosed above, or a data storagemedium (e.g. semiconductor memory, magnetic or optical disk) having sucha computer program stored therein.

In block diagrams, illustrated components are depicted as discretefunctional blocks, but embodiments are not limited to systems in whichthe functionality described herein is organized as illustrated. Thefunctionality provided by each of the components may be provided bysoftware or hardware modules that are differently organized than ispresently depicted, for example such software or hardware may beintermingled, conjoined, replicated, broken up, distributed (e.g. withina data center or geographically), or otherwise differently organized.The functionality described herein may be provided by one or moreprocessors of one or more computers executing code stored on a tangible,non-transitory, machine readable medium. In some cases, third partycontent delivery networks may host some or all of the informationconveyed over networks, in which case, to the extent information (e.g.,content) is said to be supplied or otherwise provided, the informationmay be provided by sending instructions to retrieve that informationfrom a content delivery network.

Unless specifically stated otherwise, as apparent from the discussion,it is appreciated that throughout this specification discussionsutilizing terms such as “processing,” “computing,” “calculating,”“determining” or the like refer to actions or processes of a specificapparatus, such as a special purpose computer or a similar specialpurpose electronic processing/computing device.

The reader should appreciate that the present application describesseveral inventions. Rather than separating those inventions intomultiple isolated patent applications, applicants have grouped theseinventions into a single document because their related subject matterlends itself to economies in the application process. But the distinctadvantages and aspects of such inventions should not be conflated. Insome cases, embodiments address all of the deficiencies noted herein,but it should be understood that the inventions are independentlyuseful, and some embodiments address only a subset of such problems oroffer other, unmentioned benefits that will be apparent to those ofskill in the art reviewing the present disclosure. Due to costsconstraints, some inventions disclosed herein may not be presentlyclaimed and may be claimed in later filings, such as continuationapplications or by amending the present claims. Similarly, due to spaceconstraints, neither the Abstract nor the Summary of the Inventionsections of the present document should be taken as containing acomprehensive listing of all such inventions or all aspects of suchinventions.

It should be understood that the description and the drawings are notintended to limit the invention to the particular form disclosed, but tothe contrary, the intention is to cover all modifications, equivalents,and alternatives falling within the spirit and scope of the presentinvention as defined by the appended claims.

Modifications and alternative embodiments of various aspects of theinvention will be apparent to those skilled in the art in view of thisdescription. Accordingly, this description and the drawings are to beconstrued as illustrative only and are for the purpose of teaching thoseskilled in the art the general manner of carrying out the invention. Itis to be understood that the forms of the invention shown and describedherein are to be taken as examples of embodiments. Elements andmaterials may be substituted for those illustrated and described herein,parts and processes may be reversed or omitted, certain features may beutilized independently, and embodiments or features of embodiments maybe combined, all as would be apparent to one skilled in the art afterhaving the benefit of this description of the invention. Changes may bemade in the elements described herein without departing from the spiritand scope of the invention as described in the following claims.Headings used herein are for organizational purposes only and are notmeant to be used to limit the scope of the description.

Further embodiments according to the present invention are furtherdescribed in below clauses:

1. A method of optimizing a metrology process, the method comprising:

obtaining measurement data from a plurality of applications of themetrology process to a first target on a substrate, wherein:

-   -   each application of the metrology process comprises illuminating        the first target with a radiation spot and detecting radiation        redirected by the first target;    -   the applications of the metrology process include applications        at either or both of a) plural positions of the radiation spot        relative to the first target, and/or b) plural focus heights of        the radiation spot; and    -   the measurement data comprises, for each application of the        metrology process, a detected pupil representation of an optical        characteristic of the redirected radiation in a pupil plane; and

determining either or both of an optimal alignment and an optimal focusheight based on comparisons between the detected pupil representationsin the measurement data and a reference pupil representation.

2. The method of clause 1, wherein either or both of:

an alignment associated with the detected pupil representationdetermined to be the most similar to the reference pupil representationis determined to be the optimal alignment; and/or

a focus height associated with the detected pupil representationdetermined to be the most similar to the reference pupil representationis determined to be the optimal focus height.

3. The method of clause 1 or clause 2, wherein the comparisons betweenthe detected pupil representations and the reference pupilrepresentation comprise calculating a degree of similarity between eachdetected pupil representation and the reference pupil representation.4. The method of any of clauses 1-3, wherein the reference pupilrepresentation is generated using a simulation of redirection ofradiation from the first target.5. The method of any of clauses 1-4, wherein the reference pupilrepresentation is generated using a detected pupil representationobtained by a previous application of the metrology process.6. The method of clause 5, wherein the previous application of themetrology process was applied to a first target at either or both of anoptimal alignment and an optimal focus height.7. The method of clause 5 or clause 6, wherein the previous applicationof the metrology process comprises an application of the metrologyprocess to a second target, the second target being larger than thefirst target.8. The method of clause 7, wherein the first target and the secondtarget each comprise a periodic structure defined by a unit cell thathas the same periodicity in at least one direction.9. The method of clause 8, wherein the unit cell of the first target isidentical to the unit cell of the second target.10. The method of any of clauses 7-9, wherein:

the substrate comprises one or more device regions configured tocomprise device

structures and one or more scribe lanes positioned outside of the deviceregions; and the first target is located in one of the device regionsand the second target is located in one of the scribe lanes.

11. The method of any of clauses 1-10, wherein the reference pupilrepresentation is generated using machine learning based on pluralprevious applications of the metrology process with differentalignments, different focus heights, or both.12. The method of any of clauses 1-11, wherein:

the method is applied to each of a plurality of first targets atdifferent locations; and

the plurality of positions of the radiation spot used when applying themethod to one first target is selected using an optimal alignmentobtained by applying the method to at least one other first target.

13. The method of any of clauses 1-12, further comprising analyzing adetected pupil representation corresponding to either or both of anoptimal alignment and/or an optimal focus height determined by themethod of any of clauses 1-12 to obtain a value of a parameter ofinterest.14. The method of clause 13, wherein the parameter of interest comprisesone or more of the following: overlay, critical dimension.15. A computer program product comprising a computer non-transitoryreadable medium having instructions recorded thereon, the instructionswhen executed by a computer implementing the method of any of clauses1-14.16. A system comprising:

a computer system; and

a non-transitory computer readable storage medium configured to storemachine-readable instructions, wherein when executed, themachine-readable instructions cause the computer system to perform themethod of any of clauses 1-14.

17. A metrology apparatus for measuring a target on a substrate, themetrology apparatus configured to perform the method of any of clauses1-14.18. A system comprising:

a metrology apparatus configured to provide a beam of radiation onto asubstrate and to detect radiation redirected by a target on thesubstrate; and

the computer program product of clause 15.

19. The system of clause 18, further comprising a lithographic apparatuscomprising a support structure configured to hold a patterning device tomodulate a radiation beam and a projection optical system arranged toproject the modulated radiation beam onto a radiation-sensitivesubstrate, wherein the lithographic apparatus is configured to control asetting of the lithographic apparatus based on information obtainedusing the metrology apparatus and the computer program product.

As used throughout this application, the word “may” is used in apermissive sense (i.e., meaning having the potential to), rather thanthe mandatory sense (i.e., meaning must). The words “include”,“including”, and “includes” and the like mean including, but not limitedto. As used throughout this application, the singular forms “a,” “an,”and “the” include plural referents unless the content explicitlyindicates otherwise. Thus, for example, reference to “an” element or “a”element includes a combination of two or more elements, notwithstandinguse of other terms and phrases for one or more elements, such as “one ormore.” The term “or” is, unless indicated otherwise, non-exclusive,i.e., encompassing both “and” and “or.” Terms describing conditionalrelationships, e.g., “in response to X, Y,” “upon X, Y,”, “if X, Y,”“when X, Y,” and the like, encompass causal relationships in which theantecedent is a necessary causal condition, the antecedent is asufficient causal condition, or the antecedent is a contributory causalcondition of the consequent, e.g., “state X occurs upon condition Yobtaining” is generic to “X occurs solely upon Y” and “X occurs upon Yand Z.” Such conditional relationships are not limited to consequencesthat instantly follow the antecedent obtaining, as some consequences maybe delayed, and in conditional statements, antecedents are connected totheir consequents, e.g., the antecedent is relevant to the likelihood ofthe consequent occurring. Statements in which a plurality of attributesor functions are mapped to a plurality of objects (e.g., one or moreprocessors performing steps A, B, C, and D) encompasses both all suchattributes or functions being mapped to all such objects and subsets ofthe attributes or functions being mapped to subsets of the attributes orfunctions (e.g., both all processors each performing steps A-D, and acase in which processor 1 performs step A, processor 2 performs step Band part of step C, and processor 3 performs part of step C and step D),unless otherwise indicated. Further, unless otherwise indicated,statements that one value or action is “based on” another condition orvalue encompass both instances in which the condition or value is thesole factor and instances in which the condition or value is one factoramong a plurality of factors. Unless otherwise indicated, statementsthat “each” instance of some collection have some property should not beread to exclude cases where some otherwise identical or similar membersof a larger collection do not have the property, i.e., each does notnecessarily mean each and every.

To the extent certain U.S. patents, U.S. patent applications, or othermaterials (e.g., articles) have been incorporated by reference, the textof such U.S. patents, U.S. patent applications, and other materials isonly incorporated by reference to the extent that no conflict existsbetween such material and the statements and drawings set forth herein.In the event of such conflict, any such conflicting text in suchincorporated by reference U.S. patents, U.S. patent applications, andother materials is specifically not incorporated by reference herein.

The descriptions above are intended to be illustrative, not limiting.Thus, it will be apparent to one skilled in the art that modificationsmay be made to the disclosure as described without departing from thescope of the claims set out below.

1. A method of optimizing a metrology process, the method comprising:obtaining measurement data from a plurality of applications of themetrology process to a first target on a substrate, wherein: eachapplication of the metrology process comprises illuminating the firsttarget with a radiation spot and detecting radiation redirected by thefirst target; the applications of the metrology process includeapplications at either or both of a) plural positions of the radiationspot relative to the first target, and/or b) plural focus heights of theradiation spot; and the measurement data comprises, for each applicationof the metrology process, a detected pupil representation of an opticalcharacteristic of the redirected radiation in a pupil plane; anddetermining either or both of an optimal alignment and/or an optimalfocus height based on comparisons between the detected pupilrepresentations in the measurement data and a reference pupilrepresentation.
 2. The method of claim 1, wherein either or both of: analignment associated with the detected pupil representation determinedto be most similar to the reference pupil representation is determinedto be the optimal alignment; and/or a focus height associated with thedetected pupil representation determined to be most similar to thereference pupil representation is determined to be the optimal focusheight.
 3. The method of claim 1, wherein the comparisons between thedetected pupil representations and the reference pupil representationcomprise calculating a degree of similarity between each detected pupilrepresentation and the reference pupil representation.
 4. The method ofclaim 1, wherein the reference pupil representation is generated using asimulation of redirection of radiation from the first target.
 5. Themethod of claim 1, wherein the reference pupil representation isgenerated using a detected pupil representation obtained by a previousapplication of the metrology process.
 6. The method of claim 5, whereinthe previous application of the metrology process was applied to aninstance of the first target at either or both of an optimal alignmentand/or an optimal focus height.
 7. The method of claim 5, wherein theprevious application of the metrology process comprises an applicationof the metrology process to a second target, the second target beinglarger than the first target.
 8. The method of claim 7, wherein thefirst target and the second target each comprise a periodic structuredefined by a unit cell that has the same periodicity in at least onedirection.
 9. The method of claim 8, wherein the unit cell of the firsttarget is identical to the unit cell of the second target.
 10. Themethod of claim 7, wherein: the substrate comprises one or more deviceregions configured to comprise device structures and one or more scribelanes positioned outside of the device regions; and the first target islocated in one of the device regions and the second target is located inone of the scribe lanes.
 11. The method of claim 1, wherein thereference pupil representation is generated using machine learning basedon plural previous applications of the metrology process with differentalignments, different focus heights, or both different alignments anddifferent focus heights.
 12. The method of claim 1, wherein: the methodis applied to each of a plurality of first targets at differentlocations; the applications of the metrology process includeapplications at a plurality of positions of the radiation spot; and theplurality of positions of the radiation spot used when applying themethod to one first target is selected using an optimal alignmentobtained by applying the method to at least one other first target. 13.The method of claim 1, further comprising analyzing a detected pupilrepresentation corresponding to either or both of the determined optimalalignment and/or the determined optimal focus height to obtain a valueof a parameter of interest.
 14. The method of claim 13, wherein theparameter of interest comprises overlay and/or critical dimension.
 15. Acomputer program product comprising a computer non-transitory readablemedium having instructions recorded thereon, the instructions whenexecuted by a computer system, configured to cause the computer systemto at least: obtain measurement data from a plurality of applications ofthe metrology process to a first target on a substrate, wherein: eachapplication of the metrology process comprises illuminating the firsttarget with a radiation spot and detecting radiation redirected by thefirst target; the applications of the metrology process includeapplications at either or both of a) plural positions of the radiationspot relative to the first target, and/or b) plural focus heights of theradiation spot; and the measurement data comprises, for each applicationof the metrology process, a detected pupil representation of an opticalcharacteristic of the redirected radiation in a pupil plane; anddetermine an optimal alignment and/or an optimal focus height, based oncomparisons between the detected pupil representations in themeasurement data and a reference pupil representation.
 16. The computerprogram product of claim 15, wherein either or both of: an alignmentassociated with the detected pupil representation determined to be mostsimilar to the reference pupil representation is determined to be theoptimal alignment; and/or a focus height associated with the detectedpupil representation determined to be most similar to the referencepupil representation is determined to be the optimal focus height. 17.The computer program product of claim 15, wherein the comparisonsbetween the detected pupil representations and the reference pupilrepresentation comprise calculating a degree of similarity between eachdetected pupil representation and the reference pupil representation.18. The computer program product of claim 15, wherein the referencepupil representation is generated using a simulation of redirection ofradiation from the first target.
 19. The computer program product ofclaim 15, wherein the reference pupil representation is generated usinga detected pupil representation obtained by a previous application ofthe metrology process.
 20. A system comprising: a metrology apparatusconfigured to provide a beam of radiation onto a substrate and to detectradiation redirected by a target on the substrate; and the computerprogram product of claim 15.